Valve pretty much went above and beyond when it comes to sale of Oblivion. They contributed either code or advice (IIRC) to the OBSE project to make it work with the Steam version of the game.
lol, the lack of a functioning OBSE on steam was probably stopping people from buying the steam version, great move on their part.
On a related note, this probably wont be great news to many here, but bethesda games have a pretty good discount on steam right now in honor of fallout new vegas. Goty Oblivion is only $8.50, Morrowind is only $5, and fallout $25