Isn't most of their DRM encryption based? I mean, it's not all that restrictive, and it's quite possible that simply seeing the code for the DRM would not be enough to circumvent it. I think other companies would have a problem, but I don't think valve is probably one of of them. I could be wrong though.

Preorder delivery is encrypted, yeah. However, they also have bits in Steam-delivered games that can check for a valid Steam session, terminate the game session if the Steam session is lost, and the like. I don't think they'll be shipping some of those components because they wouldn't want to enable people building custom Steam installs that just stub out the DRM features.

While I agree that they will definitely not be releasing source for Steam, I doubt its for DRM reasons. As far as I can tell steamapi.DLL DRM is pretty thoroughly owned.