The AUX-IN input is basically a 3.5mm stereo jack. I'm not sure if the standard non-Shaker audio unit has an AUX button. If it does, there's a good chance it has the AUX input. This is not the same as a data interface for the iPod where you can control the iPod using your audio receiver.
What if I don't like the song that is being played on the playlist? I'd still have to FF it to the next song on the list, like my Winamp MP3 player (which has been around for 10+ years). My commute is only 30-45 min tops (about 10-20 songs tops). I guess the iPod is better than a Shaker for those who have 10+ hour commutes
iPods are superior to CD MP3 players in the respect that you can add/delete songs and organize them more easily. The seeking is also faster since you are dealing with a HD or solid-state components instead of an optical disc. I don't see the 30GB HD capacity as being a major selling point because with 6 discs and 11 hours of audio per disc, it should be more than enough for the daily commute. The best part is that on a 2007 GT, you can plug in your iPod if you have one, or use the Shaker (best of both worlds).
I don't have an iPod and I don't like the iPod interface. I would much rather have the Creative Zen Micro because I would use it for when i went and worked out and did other things (it has the radio and all that jazz) but it doesn't have the same hookup that all the aftermarket head units are supporting. I had an iPod, but my dorm was broken into and it was stolen and I can't justify $250+ for another iPod that I really barely used anyways.