Google has now posted advice on how to migrate to Windows 7 and keep your Picasa database, meaning that it will not need to rescan you photos and will retain all information about then including name tags. They have a method for upgrading and for a clean install of Win 7.
Basically you need to back up:
"C:Users%username%AppDataLocalGooglePicasa2"
and
"C:Users%username%AppDataLocalGooglePicasa2Albums"