The Amazon MP3 application on the Pre allows you to download and stream music files from the Amazon store. Once you find a song you can just hit “buy”, when you’ll be taken to your Amazon account for sign-in (if you haven’t already done so) and it’ll use the payment mechanism setup there. If the charge is unsuccessful, it’ll ask you to update your Amazon account and if you click the proffered link, it’ll take you to Amazon within the Web Browser app. This app works for both 3G and Wi-Fi connections and there is no noticeable difference in streaming quality between the two. 

You can play the music file in the background while you’re using other applications on the Pre. One caveat, however, is that there is no way to actually save the mp3 file itself for later offline playback. However, with an unlimited data plan this will not really be an issue, especially since the streaming is uninterrupted and high quality.