(this answer copy/pasted from another one I submitted...)
you can try several things to unlock your iPod
1) connect your iPod to the first computer you sync'ed it to. Open iTunes, then disconnect your iPod. Your iPod *should* be unlocked now
2) if you don't mind erasing all the contents of your iPod, then restore your iPod. This will remove the passcode lock and restore your iPod to original factory condition.
3) I hear that if your iPod supports disk mode - i.e. can be mounted on your Mac/PC as a USB attached drive, then it may be possible to unlock your iPod without doing a restore. This involves deleting a normally hidden file on the iPod - this file is located on the iPod in:
on the Mac, you need to run a couple commands in Terminal to [enable the Finder to display hidden files] in order to find this file on your iPod. On Windows, you can set an option in Explorer in Tools-Folder Options-View-Hidden files and folders-show hidden files and folders.
this 3rd method I've not tried myself as I don't have nano to try it