It looks like someone on the Microsoft Web site design team either goofed up or has a strange sense of humor. The folks over at VE3D got a tip that an image displaying Halo 3 as a PC game was showing up at the Microsoft Download Center.
Curious, I went to confirm it for myself, and sure enough, there it was sitting next to a link for free trials of Microsoft Game Studios games. The graphic features Halo 3 sitting on top of Zoo Tycoon and Gears of War, both of which already have PC versions.
The page I went to after clicking the link had a list of Games for Windows that was lacking in a Halo 3 listing, though. Only Halo: Combat Evolved and Halo 2 were present. That could mean that this was all just a mistake, but seeing as how the previous two games in the series have gotten the PC treatment I have a pretty strong suspicion that Halo 3 won’t be 360 exclusive forever and someone just jumped the gun before Microsoft announced it.
Is the Halo series something that you prefer playing on the PC? I had a friend who used to love the Halo: Combat Evolved port, but he kind of lost interest after the “lackluster” Halo 2 release.