How can i totally wipe my n900 with broken usb so it can be safely sent back to Nokia?

I have managed to get an external charger so i can get access to the phone to backup my the data, but how can i now wipe it clean so my data does not end up in someone else's hands.

Note: the phone also has the Nitdroid dual boot option available... if nothing is available from the maemo side is there a way i can wipe it from the Nitdroid side?