The tricky part is that if you go too fast, the dashboard widget will crash so you need to leave the song playing for about 8 seconds to pretty consistently get the download to succeed.
tell application "iTunes"
repeat while artist of current track ≤ "Zebrahead"
It's been running for about 6 hours now and we just passed John Butler Trio. I added the current track artist ≤ "Zebrahead" since they are the last artist in my list and when it is done, I would rather have it finish cleanly than just barf an error when it runs out of songs. I'm sure there's a better way to do it but this certainly works. It also lets me stop and restart it wherever I want since it doesn't really care where it starts, it just keeps going through the list.
While I have muted it during the run, it's kind of fun to let it play out loud - you get up to 8 seconds of each song in your library. It's also funny to watch my Windows Live Messenger blip change every 8 seconds - if you're on my messenger buddy list, you can watch the progress :)