[Steven Troughton-Smith] figured out how to push signed firmware through to the iPod Nano 6g. This is accomplished by modifying iRecovery to recognize the device on the USB after forcing a recovery mode reboot. So no, this doesn’t mean that it has been cracked since it checks the firmware you push and reboots if it’s not approved. But if you can figure out how to craft a custom image that passes the check you can call yourself a jailbreak author.
[Thanks RavK via NanoHack]