Why didn't Microsoft release a PC version as well, compatible with XP?
Don't they want money? Or is it that they think that Halo 3 is so damn good, PC gamers will jump in and buy an Xbox 360 console just for the sake of playing Halo 3?
The only reason that I can think of is Microsoft is trying to boost its Xbox sale (in regards to PC gamers market) by making certain games like Halo exclusive to Xbox 360. But are there really that many PC gamers buying Xbox because of Halo 3? Well, I'm not one of them.
I might be tempted to get a PS3 though, once titles like Gran Turismo, Final Fantasy, or Metal Gear came out...
Here's a product idea, why don't they invent an accessory like a mouse + gaming keyboard set (since it's gaming keyboard, it doesn't need to be full sized) that they can plug into the console, so it might be more tempting for PC gamers to buy? As Chu said, I am much more comfortable playing FPS using mouse+keyboard.