Grab Album Art From iTMS
This generous soul has provided a page where you can enter any iTunes Music Store URL and have it return the available album art, in its regular resolution. (600×600)
The source is also available if you want to see how it’s done.
Update: Continues to work with iTunes 6, and also supports video ‘poster frames’, usually the first frame. I pulled the poster frame from Beck’s ‘Pay No Mind (Snoozer)’ in iTunes 6 with no issues.