Yeah...you should be able to use jailbreakme.com to jailbreak / activate the phone once you've downgraded. After that though...the phone part will be bricked until you update to 1.1.2, for that you have to install OktoPrep, update to 1.1.2, then run the touchfree jailbreak on your Mac. After that you should be able to use approved SIM cards to get service (however you cannot unlock, although it is possible to use a SIM proxy such as TurboSIM, StealthSIM, etc to make other SIM cards work without unlocking).