I've gotten a number of emails from people who received an Upgrade version of Windows 7 in the mail and installed it on a new or formatted PC without having to resort to any tricks (like the Vista-era "install it twice" hack). If true, this does of course answer the number one remaining question about Windows 7: How do you do a clean install with Upgrade media? Apparently, with no effort at all. I will test this as soon as possible of course, but it's a very busy day so I'm not sure when I'll be able to get to it. In the meantime, it looks like we're getting some good news here.
If you have done a clean install with Windows 7 Upgrade media (i.e. there is no other OS installed on the PC at the time), please post here and let everyone know.
UPDATE: I tried this type of install in a VM and it did not work. Based on the error message I got, the Vista-era rules apply. That is, you'll have to do a stupid "install it twice" workaround as described above. Sigh.