This happend to me yesterday and it was the dumbest thing. I reset the iPod several times, restarted the computer, tried reinstalling the iPod software (of course didn't work). Finally I humbly checked all my connections and...the CD burner into which the iPod dock FireWire cable was connected had come unplugged.
So at the risk of insulting you, first make sure that everything is plugged in, not just your FireWire cables but also any FireWire peripherals in the chain.
If that simple solution doesn't work, I've had luck in this situation before by deleting the file
homedirectory/Library/Preferences/com.apple.iPod.plist
and restarting the computer.
Good luck!