This is supposedly the official Windows 7 upgrade chart from Microsoft. It looks like all Vista installs except Starter can do a 32-bit to 32-bit upgrade and keep all your files, same with 64-bit to 64-bit. Windows XP owners will have to do a clean install after backing up your files.
Most Vista users will be able to upgrade in place to the corresponding 32- or 64-bit version of 7. That should cover the vast majority of people running Vista, but if you're still on XP or you're trying to do anything out of the ordinary you'd better get ready for some pain: all those ominous blue boxes require you to back up, wipe your drive, and reinstall a totally clean copy of 7. You heard that right -- the Windows 7 installer won't even try to retain your data and programs if you're not updating from the corresponding version of Vista.