Apparently PC version of Gears of War is going to be announced at E307. It is going to carry "Games for Windows Live" banner.
The original video got pulled, but as always someone managed to save it and post it to YouTube. Very poor image quality, but some things can be seen from it. Apparently footage looks like its from a PC version. You can check it out
As we all know, Games for Windows Live titles are always Vista exclusive, or more commonly known as "Buy Vista if you want to play this game", and Microsoft will continue to do it. All those that are still on XP will have to wait for someone to "fix" this game so it will work under XP, just like Shadowrun and Halo 2.