When I connect my iPod Classic to iTunes for syncing, I automatically get an error message: "Contacts and Calendars cannot be synced on the iPod "my name". You need to turn on synchronization in iSync preferences before you can sync contacts and calendars" But I know that iSync is only for other devices and I should be able to sync my Classic from iTunes.
I just reset iSync and restored my iPod and only synced my contacts and calendars at first (I think you can do without reseting iSync). I had to restart iTunes once and synced again with all my music. It worked but it was just a hazard to put all my music back on the iPod but if you want your contacts and calendar that's what you have to do.