Your Amazon MP3 Music purchases can only be downloaded once. After you have successfully downloaded the file to your computer at the time of purchase, we recommend that you create a backup copy. We are currently unable to replace any purchased files that you delete or lose due to a system or disk error.
And I in fact just lost the drive with my mp3 collection, and I hadn't backed it up since I'd bought the downloads from Amazon.
(It's all on my ipod, I guess at some point I'll try to figure out how to extract it back from there.)