First, try to restore the iPod. You may have to let the battery drain and then charge it & connect to the updater. If it doesn't mount the first time then reset it and try to connect it again. You may have to do it several times to try and force a mounting by the updater. When I had the problem that's how I got my iPod to mount.
Here are the troubleshooting tips if you haven't already tried them (http://docs.info.apple.com/article.html?artnum=61003