Okay, so, I just tried the Half-Life games that run in the Source engine and the first Portal game on a Windows 8 machine that has never, EVER had Steam installed on it, and all of them ran absolutely perfectly. I can now confirm with absolute certainty that, at least for now, Portal, Half-Life: Source, Half-Life 2, Episode One, Episode 2, and Lost Coast are all completely DRM-free.
I also tested the Linux versions of these same games after doing a complete reinstallation of Linux Mint 15, and they all ran beautifully as well. I can't even express how happy this makes me, as these are some of my favorite games of all time.
Honestly, I think this is a mistake on Valve's part, so if you have the games, I suggest backing them up while you can. If this ISN'T a mistake, then Valve should really be advertising that they're removing DRM in their games; They'd get a ton of anti-DRM players (like I am) to grab a bunch of their games.
EDIT: It's also worth mentioning that I didn't even do an initial startup of the games as soon as they were done downloading from Steam -- I literally just downloaded them, quit Steam, and backed the folders up.
1 - Valve have often stated how they are for openness and open systems, so it most most likely by design
2 - DRM Free is only a sales gimmick that only goes so far, and quite a lot of people just don't care that much, so it depends how important it is to advertise for them or not as long as it is not part of some grander schemes of things
3 - No point doing so, as the DRM free fanatics will just say that Steam in itself is drm, and that you can not get DRM free games from them, so even with advertising this fact it will not get those people here.