Just yank it. OSX and Apple in general like to bitch at you about disconnecting. The reality is that there will only be a problem if data was in the process of being written to the iPod (in which case it will corrupt the file that was being transferred)
Once you pull it out, try plugging it back in, ideally to a different USB port to make sure everything is kosher. This is not a serious problem (yet). Don't be scared of those screens; don't let the iPod tell you what to do. I have yanked my iPod out multiple times and it really does no damage (again, so long as it isn't trying to actively write data, which, if it is not being detected at all, is not)
[EDIT]
Try looking in System Profiler and see if the iPod is coming up at all. If Finder isn't seeing it though, I doubt it will. Again, don't freak out, this happens from time to time, just unplug and replug once the iPod reboots itself.
[/EDIT]