Just had the very same problem. iPod Classic shows up on my desktop but not in iTunes (11.0.1). I tried restarting iPod while connected as suggested elsewhere. Didn't work. What did seem to work however was, while the iPod is connected (and on the desktop) restart iTunes. Once it restarted it was there and synching. I also, before restart, made sure that auto-synching was set up in iTunes preferences (under devices).
Try this. May work for you as well.