Lots of good ideas out there. My iPod classic 8GB all of a sudden stopped being recognized by iTunes 9 and the strange thing happened that my wife's exact same model would work just fine.
Long story short. I installed iTunes 220.127.116.11 (found it on brothersoft.com. That immediately recognized my iPod and the 1.0 firmware which is outdated and seems to be the root cause. After updating to the latest 1.1.2 firmware (for my particular model) it is now again recognized in iTunes 9.
I did remove ALL Apple programs from the main computer and did a total reinstall of iTunes 9 but unsure if this made any difference.
Very disappointed with Apple after this experience.
Bottom line: Try installing an other iTunes version and updating the iPod firmware and see if that fixes the problem.