iPhoneMusicCenter is a native iPhone application that allows to browse AmazonMP3 cataglog and to listen samples. With this application, you will be able to buy and download MP3 files from your iPhone/iPod.
Downloaded Musics are imported in iPod/iPhone Music Library with the Cover.
AmazonMP3 files are DRM-free and often cheaper.
It is not an Amazon project.
The application is available in Installer/Cydia (source thebigboss) in Multimedia section.