According to details found in the Steam Database brought to light in this NeoGAF thread, the new patch that rolled out for Fallout 3 looks to have removed Game For Windows LIVE features from the game. According to the thread, variables with names like ShowCDKeyOnLaunch and LegacyKeyRegistryLocation have been removed, which is a good indication that the Microsoft gaming service is no longer supported by the game.
Of course, this makes sense because Microsoft pulled the plug on GFWL awhile ago, and games such as Ultra Street Fighter IV, BioShock 2, the Arkham series, and DiRT 3. Subsequent reinstallation of the game by multiple outlets shows that Games for Windows Live prompts are no longer part of the installation process.
Bethesda has not publicly commented on any of this.
Source: Blues News