However, if you copy the image to the clipboard, you can add it to the database instead of to the track. Make sure that a track from the album is playing, and that the mini viewer window at the bottom left of the iTunes window is open. It should say "Drag Album Artwork Here". If you control-click on this window, you get one option: Paste. You can then paste in the artwork from the clipboard. It gets added to the whole album instantly, without the delay as the art is written to every file like before.
If you have CoverFlow (still available from MacUpdate, etc.), you can do a Command-C on a playing album and it will copy the artwork to the clipboard. You can guess the rest. Yeah, this will still take some time, but as CoverFlow (the old app) follows the playing track in iTunes, the artwork is already right there for you. I'm sure this is scriptable somehow. And CoverFlow (the old app) also grabs its art from Amazon and Google image searches, so it gets a lot more than iTunes does.
I don't have a functioning iPod to test this on, but a quick look around the web seems to confirm that iPods will read this new artwork database.
[robg adds: While it appeared I was able to add artwork to the database using this trick, I don't think it's quite working properly. It seems that only the currently-playing song would have the actual artwork associated with it (it shows in the Get Info window, and in the Now Playing window). Other tracks on the album had no artwork in Get Info or Now Playing (or on my iPod), even though the cover did appear in the 3D cover browser view. This was true regardless of whether I selected one or all album tracks prior to pasting in the Now Playing window. Can anyone clarify?]

