http://www.gamersquad.com/category/First-person-shooters/Bungie-Halo-3-for-PC-is-not-in-development/
Wanna know I think? Games published by Microsoft previously on the xbox will ported PC, but probably longer than the time it took them to port Halo 2 to Vista (3.5years). Bungie are busy developing Halo 3: ODST for the 360 Microsoft would prefer to focus on marketing it since the 360 market is more lucrative.
Loot at Sudeki. Its sales on the Xbox was less than sensational, still it went to the PC. Fable Gars of War is also published by Microsoft it both came out to Xbox (latter Xbox 360) PC.
U r not the only one hoping for a PC version:
http://forums.gamesforwindows.com/t/124.aspx
I'm a Halo fan and I'm crossing my fingers that it would make it to the PC.