I was playing around with the new iTunes 7 with the "Automatically download missing album artwork" option set, and I clicked on the Advanced > Get Album Artwork menu item. I noticed that the 30 second previews from the store, which I had dragged into a playlist as a wish list, were also updated with the artwork.
This means I could trigger the update on wish list tracks, then open the Info window (Command-I), go to the Artwork tab, click to highlight the artwork and copy. From there, I could and paste it into Preview, and bingo you've got artwork ... though you probably like the song anyway.
[robg adds: This technique could be useful for songs that, for some reason, just won't find artwork in the store -- even though the song itself is in the store. Just paste the image into the troublesome track's artwork, instead of into Preview. A couple people I've spoken to have seen this "no match even though they have the song" issue.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060914080227723