Thought I would just put together a list of the publishers not on GOG and the chances we will see them at some point.
EA
Why do we need them?
They hold a motherload of titles and a great many near the top of the wishlist such as System Shock, Ultima, Dungeon Keeper and Crusader: No Remorse.
What is their attitude to DRM?
Varies. At the moment they have a more relaxed attitude using SecuROM as a disc check. However C&C4 uses an always online DRM.
What are the chances we will see them?
Depends on who you talk to. Many still see them as the real evil empire. I don't think DRM is a sticking point with EA it's more about control. They have always demanded total control over their IP's and usually only deal with digital outlets on their own terms. While they are not blind to the benefits of releasing their old titles they are just as likely to make their own classic gaming service instead of signing with GOG. EA will probably wait and see how Activision fare. Pride wont allow EA to let Activision gain an advantage.
Square-Enix
Why do we need them?
Unless you have been living under a rock you should know they now own Eidos and all their IP's like Thief, Legacy of Kain, Tomb Raider, Deus Ex and Hitman.
What's their attitude to DRM?
Usually SecuROM with limited activations. In fairness though they do provide a de-authorization tool for Batman: Arkham Asylum so they are not unreasonable.
What are the chances we will see them?
DRM is unlikely a sticking point with Squeenix however regional restriction certainly is. Japanese companies are the worst for saying "No game 4 U", just look at Just Cause 2. Getting Squeenix onboard would require them to relax this attitude. Pessimists say this just wont happen.
THQ
Why do we need them?
THQ are mainly a console game producer so they don't have a massive catalogue. However they do have Relic and that means Homeworld, Dawn of War and Company of Heroes. They also have Volition which means Red Faction.
What is their attitude to DRM?
Rather good actually. They have moved to using Stardock's GOO DRM which is one of the most consumer friendly ones.
What are the chances we will see them?
I would imagine rather good actually. The fact they don't have that many big titles is probably the only reason GOG haven't gone after them aggressively. They already sell classic games like Red Faction on Impulse and Steam.
Sega
Why do we need them?
While mostly console based they ported a lot of Saturn and Dreamcast games to Windows. Lately Sega have taken the PC market seriously and now own The Creative Assembly which means the Total War games.
What's their attitude to DRM?
Steamworks. They were one of the first publishers to fully jump into bed with Valve's new system. This is both a good and bad thing.
What are the chances we will see them?
Unlikely as much as I hate to say it. Given their relationship with Valve they are likely to stay with them. They are also a Japanese publisher so see Sqeenix.
Warner Brothers
Why do we need them?
They now own all Midway's classic titles. Not only their arcade games but also forgotten gems like Psi-Ops: The Mindgate Conspiracy.
What is their attitude to DRM?
They co-published Batman: Arkham Asylum with Square-Enix which used SecuROM.
What are the chances we will see them?
That all depends on how keen they are to exploit Midway's back-catalogue. The Williams Arcade Classic packs are selling on Direct 2 Drive but I assume that distribution deal was signed before the collapse of Midway.
Capcom
Why do we need them?
Some of the biggest titles in recent years such as Chaos Legion, Street Fighter, Devil May Cry and Resident Evil.
What is their attitude to DRM?
Varies. Games purchased on Impulse use Stardock's GOO which is a good sign.
What are the chances we will see them?
I'm hopeful. GOG once asked what Capcom games we would be interested in so they are obviously asking Capcom. Out of all the Japanese publishers Capcom seem to be the most forward thinking and are clearly interested in the PC market. They were also one of the first Japanese publishers to 'get' retro gaming releasing the Capcom Generations packs on Playstation and Saturn.
Lucasarts
Why do we need them?
Where do you start? Maniac Mansion, Knights of the Old Republic, TIE Fighter, Indiana Jones and the Last Crusade and Monkey Island for starters.
What's their attitude to DRM?
Usually relaxed. SecuROM disk checks and Steam.
What are the chances we will see them?
The $64,000 dollar question. So far their attitude to their old games has been a baffling attitude of releasing some but denying all knowledge of others. They aggressively protect their IP's and release what they want on their terms regardless of what the fans actually want. SCUMMVM makes it easy for GOG to release classic Lucasarts titles but the question is do Lucasarts want to release them? They certainly don't need the money.
Valve
Why do we need them?
Half-Life and all the games that use the Source engine.
What's their attitude to DRM?
Steam and Steamworks. If you want a Valve game you have to have Steam installed.
What are the chances we will see them?
Zero.
Zenimax
Why do we need them?
Zenimax are the parent company of both Bethesda and ID.
What's their attitude to DRM?
Relaxed. Oblivion used a disk check, Doom3 was the same. Fallout 3 used GFWL.
What are the chances we will see them?
I would imagine there is really no sticking point here. DRM and regional restrictions are not that bad with Zenimax published games. Bethesda already released the first Elder Scrolls games as freeware so I would imagine getting Zenimax onboard would not be that much of a struggle.
Microsoft
Why do we need them?
Microsoft Flight Simulator, Dungeon Siege, Age of Empires, Halo.
What's their attitude to DRM?
These days it's GFWL with everything. Everything Microsoft do indirectly promotes the Xbox360.
What are the chances we will see them?
With Steve Ballmer at the helm no one can predict what the company will do. They are as unpredictable as he is. Microsoft's main interest these days is with the Xbox360 which means they are unlikely to be interested in GOG. Another company who just don't need the money.
Take Two
Why do we need them?
They now own Firaxis along with Rockstar. This means Civilization and Grand Theft Auto along with many, many others like Bioshock.
What's their attitude to DRM?
They like their fans to suffer. While Sid Meiers games have been largely free of horrid DRM the last GTA game made you jump through hoops just to play the game. Bioshock also used limited activations without a revoke tool. This was later patched out thanks to much RAGE.
What are the chances we will see them?
If they can get over their need to absolutely hate their own customers then good. Rockstar already made GTA, GTA2 and Wild Metal freeware. Sid Meier wants as many people as possible to play his games and has been rather outspoken in the past about DRM. If Take Two can release the reigns a little with DRM then there is no reason they cannot be added to GOG.

Umm, Lucasarts?

Excellent idea, trying to put all of this into a single thread, +1 for you, Delixe. Already added a link to this thread in the FAQ.

I was called away by a phone call. I did say I would add more later ;)

Phones are so 20th century.
ANYWAY BEFORE THIS THREAD GETS DERAILED

I thought Relic used steamworks on there dawn of war 2 games?

Yes they do. That was done before the shift to GOO. DOW2 also used Games for Windows LIVE. It was the exception however and most of THQ's games use GOO and GFWL.

Good list. Don't forget Take Two. Their attitude to DRM has been kinda bad. But at least not Ubisoft bad.

I really doubt that it;s the DRM issue for older games like GOG sells. Some companies just are not interested in their back catalog.
As for Lucasarts, I am betting they have a exclusive deal with Steam for on line distribution,and we won't be seeing any Lucasart games for a while.

I'd also add Ubisoft simply because of their radical change within the company towards DRM. Some are still waiting for the old Rayman and Splinter Cell games and other classics(hurr, Warrior Within)

Unlikely. Secret of Monkey Island is also on Direct 2 Drive.
Added. Can't believe I missed them.

Ubisoft are already on GOG as are 1C but neither have released anything in months.

I know, due their DRM trip, that's why. If a company changes it's standarts it's different to what it was before.

Well there was Driver Parallel Lines, but most people seemed to forget about that one fairly quickly. :P

I stand by what I said before. The only reason Driver: PL was released was because it sold so poorly and Ubisoft want to get as much money from that as possible. There is still no Splinter Cell, no Warrior Within, no XIII....

yep, and there is only one rainbow six game without the expansion and the same with ghost recon.
another publisher: Paradox Interactive