Why "force quit" won't always work
Force quit and all the various kill options won't always work. If a process is stuck in kernel mode, usually trying to access a device, no kill will, uh, kill it
Since the problem was fixed by unplugging the iPod I bet that was the situation. iTunes was stuck in kernel mode inside the iPod driver, with some arcane bug-device sleep interaction going on.
If this happens again, do a ps -aux and look at the iTunes line. If it has a U in the STAT field, it means it is sleeping in device driver code in the kernel.