Yeah, I agree with Jenox. A lot of people using computers are at businesses, and they're not going to be keen to upgrade if it isn't absolutely necessary. For at least a few years, the software they've already got will do just fine. Hell, most of the computers at my school are still running Windows 2000, and our district's network is made up of about 1,000 computers.
For personal use, I'd say the rates will increase faster since computers are coming preloaded with Vista on them, plus with all the hype and ads Microsoft is generating...