In an interview prior to the release of Halo 3 on the Xbox 360, Bungie's writing lead Frank O'Connor said:
"I can tell you for a fact that there's not one line of code written for a PC version at this point. We're not working on it, Microsoft hasn't even chatted to us about it yet. They realise the important thing is to get the 360 version at this point."
GameSpot -- More Halo 3 Q&A