A clean install is NOT an upgrade. Sure, you can backup and migrate files and whatnot, but you CANNOT upgrade from XP to 7 in the sense that you select "upgrade" from the install type.
The main difference is that you have to reinstall all of your programs.
Having said all that, you can still buy the "upgrade" version of Windows 7 and use it if you have XP, but you are still technically doing a clean install.

I think the confusion here is arising from the usage of the word "upgrade" in the contexts here.

In any case, a fresh install of a new OS is nearly always preferable to upgrading, based on my 15+ years of doing desktop support. =)