So far I think Epic are the only ones to have made a game Steam-activated after release. I'm talking about Unreal Tournament III. I got the CE (non-Steam) way back, a few year's later when the game was released on Steam (around the time Epic made the huge Black Edition update) I put my retail CD code into Steam and it accepted it! A quick run through the forums revealed that it was part of Epic's surprise update to make everything work through Steam.
Oddly enough, after that Epic moved on to GFWL.
Not odd at all for them to jump to G4WL, given the success they had w/ Gears of War X360.
And that they are pretty much buddies w/ Microsoft.
Coming back to Borderlands 2, I'm glad it's bound to Steam and not anything worse. At least Steam has a functioning set of online features.
There's a fair amount of games that had different DRM at Retail [Non-Steam Version], but can additionally as an option be also be activated on Steam.
UT3 is obviously one, as you stated.
Sup Com: Forged Alliance can be optionally activated on Steam.
Doing so, also nets you the original Sup Com, BTW.
Dark Messiah of Might & Magic - Single Player Portion can be optionally activated on Steam.
X3: Terran Conflict, as well.
Prey is another.
Borderlands: GOTY Edition is another.