It's actually been announced by some other developer recently that a port to PC/Mac is underway.
I've played a little of Halo in the store, and it's...well...it's my personal opinion that the X-box is a monstrosity (and the controller too), so the experience was less than enjoyable. The small amount I played it didn't give me much in the way of the story the game's supposed to have, so I can't judge on that.
Gameplay with a controller feels clumsy to me. I think the game in it's current form would be more fun on a computer, and I KNOW that th egame in it's original, un-bastardized form ('scuse my french) would be even more fun.
Now that it's officially coming to the computer, I think it's a cool game, but I'm not especially impressed. It was originally being developed for the computer and then got watered down for a console when Bungie got "acquired".
*shrugs*
-Joy