Well without making an "I told you so" speech, I will state it once more, on-the-record, that Call of Duty 4 WILL be available on the PC. In
Interview with the man himself, Grant Collier, I learned almost a full year ago that CoD4 was being developed for PC fans.
Furthermore, I would wager my personal gaming PC (a new XPS 710 fully-loaded w/ dual 512 MB GeForce 8800's) that CoD4 is being DESIGNED for Vista. Whether or not it will have DX10 support is another story, but Infinity Ward is well aware of the new technologies, and will make sure it runs perfectly on Vista.
So please, don't feel obligated to e-mail me with this question anymore. I am answering it here; Call of Duty 4: Modern Warfare will be available for PC. It is likely going to be made for PC and Consoles (Xbox 360, PS3), and probably not for the Wii - but that is just speculation. All I can confirm is that it WILL be made for PC's.