Running Windows Vista? If you have Windows Vista, you can purchase Windows 7 Upgrade versions. You can do a clean install (back up your files, clean install, and reinstall your applications) or an in-place upgrade (Windows 7 installs over Windows Vista).
Running earlier versions? If you have Windows XP or Windows 2000, you can purchase Windows 7 Upgrade versions. But you must back up your files, clean install, and reinstall your applications.
So yes, clean installs with upgrade media will be possible.
------------------------------Desktop: Windows 7 Professional 64-bit; Intel Q6600 CPU; E-VGA 780i SLI motherboard; E-VGA E-GeForce 8800GT; OCZ Vista 4GB dual-channel kit; Ultra X2 750W power supply; 2 x Seagate Barracuda 7200.11 500GB in RAID 0. Laptop: Acer Aspire 8730-6314;
Reply to Zoron
I just intalled the Win 7 Release Candidate a few days ago. Had Vista on the pc - first I backed everything up, then tried to install Win 7 as an upgrade - it went through everything, but eventually failed. It also gave me the option to to a clean install, so I tried that and am typing this reply on Win 7 Ultimate RC right now. Beautiful!
So yea, with the info Zoron provided and my experience w/ the RC, it looks like clean install will be an option (and based on my experience with all prior versions of Windows - clean install is much preferred).