BioShock 2 no longer uses Games for Windows Live, according to ShackNews. 2K Games has switched Irrational's game over to Steamworks and has made the Minerva's Den DLC free to current owners.
Multiplayer now uses Steam's Steamworks, and all the multiplayer DLC packs have been made free to everyone. People who buy BioShock 2 now via Steam will need to buy Minerva's Den separately, but if you already own it, you get the DLC free of charge. If you purchased a boxed copy or digital copy of BioShock 2 from another digital distributor, you can also activate that key on Steam to add the game plus Minerva's Den and the MP DLC to your library, according to 2K.
One caveat to the changes is that saves, achievements, and multiplayer progress from the GFWL version won't carry over. Steamworks support also brings a handful of new features including official controller support, Steam achievements, and Big Picture mode support. The SecuROM DRM is also gone, which may be the greatest change of all.