I sync my iPod with my Mac. All files I ripped from CDs are put into MP3 format, like ssmumich stated.
Apple gave up on "Mac only / PC only" iPod builds when they came out with the third generation iPod since people were just stealing the software if they switched operating systems. When I got mine, direct from the Apple Store in Rockingham Mall, the box states "for Mac and Windows".
I believe files bought off iTMS are in AAC format for security purposes.
