Dev Team released ultrasn0w 1.2 last week for unlocking iPhone 3G/3GS on iOS 4.1 / 4.2.1. This method involves updating iPhone baseband to 06.15.00, which is the same as iPad baseband.
Now, when you update to the iPad baseband, you cannot restore to the official firmware back via iTunes, as the baseband cannot be downgraded to 05.14.02 or 05.15.04. So, if you want to restore iOS, you need to build a custom firmware via PT/redsn0w.
Earlier today, @sherif_hashim posted steps to restore to the custom iOS:
posted by sherif_hashim:
to restore ur iphone while already on 6.15.00 BB:
A) if u used pwnage tool then u ll have to create a /new/ custom 4.1 or 4.2.1 and say /no/ when u are offered the BB update to 6.15.00 since u already have it, then build the custom ipsw and restore to that
B) if u r using redsn0w, then:
B-1) if u are currently on 4.1, then u can restore to official 4.1 again, and when u see error 1015 pop-up, u kick ur device out of recovery using umbrella, use redsn0w and /don’t/ check “ipad BB update” because u already have that
B-2) if u are currently on 4.2.1 then u ll have to restore back to 4.1 (because 4.2.1 will not allow u to kick the phone out of recovery when 1015 occurs due to the silly BB / iOS mismatch check added there) , then do the same steps as B-1, if u want to go again to 4.2.1 your only way is to use a custom 4.2.1 done by pwnage tool in the same steps mentioned in A
n.b of course saved shsh for the 4.1 ipsw is an important prerequisite for iphone 3gs to restore back to that
Also, @MuscleNerd said that there will be some hack to downgrade the iPhone 3G/3GS baseband! Stay tuned for news!
Subscribe to my feeds and follow me on Twitter to stay updated 😉