you can still use the phone with AT&T or get an updated sim unlock..... but a baseband downgrade isn't available yet...only way to downgrade is with a hardware downgrade (remove and reprogram) I've heard people doing this but don't know the validity of it....
its been almost a week ive not been able to use my i - daym - phone lolz .. and im pissed cuz u cant downgrade the baseband 02.28.00 i havent found a way out and request iphone dev team to consider the unlock asap