When I look at my Music Library with Artwork enabled, I see for CDs (albums), which have different artists, an artwork for every title. This is opposed to albums with one artist (the usual situation), where only one artwork is shown and the titles are grouped together right of it's artwork.
The artwork on albums with different artists is split because the artwork shows album and artist below the artwork, which is different for each title.
Same later on on iPhone and iPod ... ;(
Examples are (titles from movies): My Best Friend's Wedding Grease Best of James Bond ...