Look at the PC sales for the 2 Halo titles that came out.  I really don't think it's that big of a deal.  This is not a statement about Halo the game, or PC users or whatever, just doesn't seem like the PC crowd has ever cared for Halo much (Based upon admittedly personal, anecdotal experience among my PC friends, and VGChartz numbers).

It will move 'some' amount of units, but I don't expect it to be very significant.

EDIT: I'll say this though, give it local server options, and maybe even allow them access to map making/mod tools (or at least allow them to be made and implemented), and that could change entirely.  But, it'd have to be a pretty significant and deep level of access to go up against the current powerhouses of modding heaven on offer.

I was joking.  Of course sales will be much less on pc 

True, but there are ways they could make it more attractive for PC users.  I edited that post a ton of times heh.