Point of aggravation, though: searching for music in the app only displays CDs, which seems somewhat backward. The mobile site for iPhone lets you search digital downloads, although you can't buy them (you have to add them to your wish list for later purchase); browsing in Safari on iPad is both better and worse, as it lets you get all the way to the point of buying an MP3 track before failing to download the helper app that in turn would download the track. Memo to Amazon: You probably want to fix that as soon as you can. Update: If you decline the helper app, you can actually download the MP3 track in the browser on the iPad -- but then what? Goodreader will open it, which is nice and all, but it's a bit awkward to do anything at that point except email it to yourself.
Would it be so difficult to include the option for MP3 purchase and simply email the user a download URL? Granted, there might be some hiccups for those iPad users who rarely/never sync their devices, but c'mon, it's 2010 already. Even though you can access the MP3 store via Mobile Safari, there's obviously no way to get music downloads into iTunes except through the iTunes Store or via tethered sync.
Amazon also delivers the Kindle and IMDB apps for iPad, just in case you felt like doing some reading or movie searching along with your shopping trip. All three apps are free.