I have been using a Windows XP PC to edit the tag info and album art on my pre-existing music in my playlist, and just when I have album art downloaded and synced to the ipad 2, I try to sync it to my Windows 7 computer, thinking the album art and tag info would sync to the Windows 7 computer, but, no go. The album art goes away and the purchased music disappears. When I go back and sync it to my Windows XP computer, it comes back, except for the purchased music.
I finally choose all my purchased music and add it to my playlist which solves one problem, but the album art work won't sync between computers. I finally copy the files in the c:\documents and settings\<username>\my documents\my music\itunes\music and the downloaded album art folders from the Win XP computer to the Win 7 computer and that solved the other problem.
You'd think that Apple would compensate or fix iTunes to solve this problem.
I just bought an app called Flick Music, which lets you choose album art on the fly. Apple should incorporate this feature into their default music app, or use Flick Music as their default music player.