This is also the reason I might have to get the boxed pack... I ALWAYS do a clean install, and it would be a pain to have to reinstall Leopard, then Snow Leopard...
So many people get this wrong every time a new version of Mac OS X comes out. The upgrade DVD will allow you to do a full erase, format and install. You do not need to first do a clean install of Leopard then install Snow Leopard.
Once SL sees the install of Leopard then you go into Disk Utility and you can fully erase the HDD and begin a clean install of Snow Leopard. The only difference between the upgrade DVD and the full version is if you install a new drive on your Mac you will have to install Leopard first then Snow Leopard.