(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 [http://support.apple.com/kb/HT1339?viewlocale=en_US|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:
iPod_Control/Devices - in that directory will be a file named _locked. You want to delete that file (or rename to _unlocked)
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