I don't normally regurgitate GTA news, but this is relavent to the site.
According to a news post on the GTA IV Steam page, the game will no longer use Games for Windows Live and previous save files will still be compatible with the new update. This hopefully means that the update will also remove the save file check that prevented players from sharing save files while GFWL is enabled. Currently, saves files from other players can only be loaded if you use a mod to disable the save file check, such as XLiveLess. Hopefully XLiveLess game and saves and the new post-GFWL game and saves will also be compatible with each other.
The update is scheduled for March 19.