Grant17:The files are MP4s using the AAC.cokemaster: All the MP3's imported into Itunes generally stay as MP3 files.
Are you saying that tracks purchased from the iTunes music store get downloaded as MP3s?
AAC is NOT Apple's format. God it annoys me when people say this.
AAC stands for Advanced Audio Codec and was developed by DOLBY. Apple only use this format for audio files in iTunes and wraps them in MP4a. This is the same as DivX not owning AVI. AVI is a wrapper for a number of codecs with DivX being just one of these - WMV is another incidentally.
FairPlay, Apple's DRM, is then applied to the iTMS bought files to satisfy the music industry's fears.
Incidentally, ripping a CD into iTunes doesn't apply the DRM to the files.