My Ipod Classic had the same problem after I updated to iTunes 9. I discovered it was a problem with the software version on the iPod; not so much iTunes 9. I think this is why only some of us are suffering this problem. The catch is, you need a successful sync to update the iPod software.
If you happen to have a second computer with iTunes 8.2 or earlier, you can use that, but if you are like 99% of people with the same version of iTunes on all computers, you need to rollback to 8.2, update the iPod, and then reinstall iTunes 9. It took me about 30 minutes to get my iPod classic working with iTunes 9 using the following steps:
1. Uninstall the iTunes 9 application as far as step 13 on this help file
http://support.apple.com/kb/HT1224
(this will remove the application, but leave the music and your libraries intact)
2. Download and install iTunes 8.2. I found a site with an old version using a google search
http://en.kioskea.net/telecharger/telecharger-241-itunes-for-mac
3. After version 8.2 is installed, start iTunes by holding down the OPTIONS key on your keyboard and use one click (don't double-click) on your iTunes icon (if it doesn't work, just quit iTunes, and try again...you will get it eventually). A window will pop-up enabling you to create a new library. Do that, and give it a random name (you won't use this library again).
4. iTunes will start (without any of your original library showing, but don't panic; it's still there), and your iPod should now connect. Choose not to transfer any files, or authorisation, however, take the option to update the iPod software.
5. Once the software is updated and has restarted, eject the iPod (no need to complete the sync)
6. Perform a normal mac software update to reinstall iTunes 9.
7. When starting iTunes 9 for the first time after reinstalling, hold down the OPTION key on your keyboard and use one click on the iTunes icon; pick the CHOOSE LIBRARY option. Open your original library that has all your music, and you should be back to where you want to be; your iPod syncing as normal. Next time you open iTunes, it will open to the correct library. The use of the OPTION key is only needed the first time.
8. Hopefully, stop banging your head against your desk.