Lexus' website GS specifications list the Mark Levinson sound system as an optional add-on, replacing the standard 6-disc CD changer. The Mark Levinson sound system can play regular audio as well as MP3s and can play both CDs and DVDs and is also a 6-disc changer. This essentially equals to about 27 gigabytes of mp3 music, which is a little less than the standard 30GB Ipod.
This essentially equals to a system that can let you play nearly as much music as a regular ipod, does not require extra power or carrying space, and still retains a casette deck in case you find that rare music tape (I still have some old tapes. I want to try converting them to digital music someday, but that's later).
Of course, all this costs extra money (although if you're getting a GS, chances are you can well afford it), and does not offer you has much space as a 80GB ipod does, for the extreme music fans out there. (I say extreme cause do you really need 20,000 songs to listen to on a drive? Just a personal thought)