So, it turns out that if you...
buy and MP3 on the Zune Marketplace,
delete the MP3
You can not restore the MP3 -OR- re-purchase it.
When you go to buy the song again, the price shows up as "restore", but then the download fails.
This is biting me right now as two initiall downloads failed, and now I have no way of getting the tracks. After an hour, the Zune customer support representative confirmed with me that this is in fact the case.
bug.
fix it.
(p.s. for my specific situation, the initial download failed, so I never even "lost" the MP3.)