This may actually be two questions in one. I've recently bought a Logitech Squeezebox Duet and as a result have noticed that a large amount of the album artwork in my iTunes library is either missing or just completely wrong.
So firstly is there an application that will perform a mass/bulk download of artwork for an iTunes library.
Secondly the controller handset for the Duet displays the album art for the track that is currently playing (and thumbnails of album art when browsing). For quite a few albums that do have correct artwork in iTunes, the art doesn't show up on the Squeezebox controller.