Was the iPod previously matched with a different PC or Mac? An iPod can only sync with one iTunes library at a time, so if this was the case you could be having problems from that. You could do a "restore" on the iPod and if it appears in iTunes after that, then you can sync.
http://www.apple.com/support/ipod/five_rs/Use the above site to troubleshoot the problem - start from the left (i.e. "reset") and work through the options, though - don't just jump to "restore" if one of the lead-up options works for you...
