1 month is too much, the story of the game couldn't take more than 2 or 3 days tops to complete and MP would be useless until the game was actually released except for local play. For a legitimate standard copy I could see paying $100 to have about a week early and play through story about 2 times just to catch or get stuffed I missed the first time just like you do in a movie you watch it 1 more time and realize something new or something makes more sense.
I bet MW2 will be online though about 2 weeks before street date and I don't know how bad the temptation will be to download it for me...I mean there really is no incentive to pre-order or purchase aside from wanting to show developers appreciation for the game.