I noticed something odd this afternoon - the newest release on Steam was an ancient game, Aquanox, available alone or in a bundle with its sequel.
Its price was 4.99, just below GOG's. (I still find GOG's version superior due to the extras, lack of DRM, and commitment to do what it takes for it to run on modern hardware, though.)
Is this a curiosity, or is Steam trying to tap into the market GOG built up?

There is no competing with GOG :)
Man every time i open my account page it's like i'm 12 again....on steam i feel 29.
GOG guys did an amazing job with this site and I hope they will continue releasing titles that made us who we are today...Nostalgic fetishists with serious aversion to consoles :)

Steam actually carries several of the games that GOG does. My guess is that the publishers who are giving permission for GOG to sell these games are giving the same permission to Steam and possibly other download services in the hopes of hitting a wider customer base. I doubt that Steam has the specific goal of out-competing GOG, or it would be doing a lot more than just stocking Aquanox, MDK, and The Longest Journey.
Remember, Steam is an extension of Valve, and was originally created solely to sell Valve products. I don't think the people who run it plan to travel too far from the tree in that respect, although they evidently won't shy away from the possibility of making a profit off of some old IPs.

Let's be hones here. GOG profits probably just pay the rent and some of the wages at CD Projekt, What will always bring in the big money is the success of their games like Witcher and Witcher 2. If Witcher 2 doesn't sell well, an EA or Valve might came along with a ton of money and successfully buy GOG.
Even if they are making $1 profit on each sale, I doubt they're bringing in more than a couple thousand a week.

True - I am not actively purchasing on GOG.com, then again, I have never purchased from Steam. This is my first month of uncappe internet! Steam managed to cap my 500MB just with adds and updates. Then there's the DRM issues. I travel a lot and requiring internet access to play a games does make me feel like being stuck to a fetus.
If GOG.com keep this going not making profits it will fold. this is the rule of a shareholder. The world we live in, is all about oil... no, wait, profit.
I hope GOG.com stay alive. GOG.com and Abandonia are my favourites. Steam is nice to watch trailers. GOG.com is small enough with the minimum information required to make a purchase.
Rock on GOG.

I just wanted to add that Steam does not require Internet access to play games. To download them, yes for sure, but not to play them... That's what the "offline mode" is for!
Please note that I'm by no mean a big fan of Steam or DRM though. GOG is the best in that regard!!!

There are a few games which require internet access to play, but the vast majority of games can be played offline as you said.

I also want to let NJK know that you can turn off automatic updates if you don't want it downloading in the background.

And a 500 mb cap? That sounds painful. I'm pretty sure my roommates and I hit that just about every day.