It's not true that MP3 songs on Amazon are at a lower bitrate than music at the iTunes Store. The difference is between 256 kbps AAC at the iTunes Store and 256 kbps MP3 at Amazon. At this bit rate the difference between the quality of AAC and MP3 is negligible. And both come without DRM. Indeed, because Universal was trying to pressure Apple into permitting variable pricing, they offered their catalog first to Amazon at the higher bitrate (when the iTunes Store only had 128 kbps AAC), and DRM free. The result was less than they hoped. Apple did eventually accede to a variable pricing structure but got the DRM stripped out, which was their goal all along. And the result of variable pricing has been less than stellar, as download music sales have slowed since the new price structure went into affect. This may level out eventually as people become used to it, but it is not at the moment a revenue neutral strategy.

There's a newer version of the script released on the author's website: http://www.advantageousmp3.com/mac/

This release primarily adds compatibility with iTunes 8.1