Jailbroken means you can use a 3rd party installer and pretty much do what you want. Unlock, unlocks the baseband so you can use any network. I'm not up to speed on the ins and outs with the latest firmware, though. I have a 2G U&J on 3.0, and a 3GS o2 on 3.1. Fairly sure 3.1 added some server checks when installing firmware.