I've gotten an unusual number of questions lately about whether it will be possible to use an Upgrade version of Windows 7 to perform a clean install, as is possible with Windows Vista (a process I validated with both the RTM and SP1 versions of Windows Vista ... I will do so with SP2 as well, once that media becomes available).
I suspect it will still work. But there's no real answer to this question yet because, well, there's no retail Windows 7 media to test. In order to see whether this works, I will need a copy of some Upgrade version of Windows 7. And Microsoft doesn't make that code available until it completes development of the product. Rest assured, I will be testing this process--both with Windows 7 and Vista SP2--as soon as the relevant code is available.