Check for updates. There was apparently a bug with the very 1st iTunes 6 which came out. Use Software Update to check. I haven't had any problems with iTunes 6 other than initially only having QuickTime 6. In order to use the extra functions of iTunes 6 you need QuickTime 7. Perhaps you could downld Audion (click on link) which is now free to check your mp3 files are ok. You may have downlded a bad batch. Just a thought!

There's some files such as Ogg & some mp3's (eg: mp3pro) iTunes won't import but Audion will play.