MacRumors has written an article about Apple’s Digital Rights Management (DRM) system and how it is using protected AAC for iTunes Music Store downloads.
“Burning a Protected AAC to a CD strips all encoding and DRM. That CD can then be used as any CD song is used. The quality of the song on the CD is identical to the AAC version. However, then ripping the song into MP3 or AAC will result in loss of some quality. While ripping a song into any lossy compression format will result in loss of quality—recompressing these previously compressed songs may exaggerate the quality loss. Your results will vary depending on the exact piece of audio. Anecdotal evidence suggests re-ripping into AAC yields better quality than re-ripping into MP3.”