I have successfully created my own custom firmware and loaded it onto a phone. But I was wondering why I didn't need to sign the firmware before loading it, or was it signed with a debug key similar to when you test your own android app?
AFAIK the N1 doesn't require signed firmware, at least not once you unlock the bootloader -- it's a dev phone. Unofficial confirmation:
I specifically purchased a Nexus One because it supports the ability to flash the phone with unsigned firmware.