buy on pay as you go, jailbreak the phone with quickpwn, update via cydia(on the handset) on the 31st
easy as that.... just dont update to the apple 2.2 firmware via itunes or you cant unlock it at all.
or buy one off fleabay already jailbroke and update on the 31st via cydia (cydia is an app on the handset, same as installer and allows you to add any of the 1000's of applications on the handset, that apple wont let you if you dont jailbreak it...