This post has the details.
One ‘interesting’ thing is that the upgrade paths only explicitly detail how to do it if you connect your phone to a computer using Vista or Windows XP. There is no Windows 7 path, in fact, the fine print explicitly says that you can’t use either of the download tools with Windows 7 (my guess is that it required too much work to create a Windows 7 specific download tool so soon to when Windows 7 Phones will be available anyway).
I wonder what would happen if you set the Vista download tool to run under Windows 7 in Vista compatibility mode and tried to upgrade. An interesting rhetorical question. It would be foolish to actually try this, of course, since you risk bricking your phone. Yep, interesting rhetorical question.