If you have updated your iPhone 3G(S) baseband to iPad baseband 06.15.00 in order to unlock it on iOS 4.2.1 and now you are having issues while restoring the device, Sherif Hashif has posted a simple method which we are going to share with you people.
In case you want to restore iPhone 3GS back to iOS 4.1, you must have saved SHSH blobs for 4.1 IPSW.
A. If you are on Mac and use PwnageTool: If you used pwnage tool then you will have to create a “new” custom 4.1 or 4.2.1 and say “No” when you are offered the BB update to 6.15.00 since you already have it, then build the custom ipsw and restore to that.
B. If you use(d) Redsn0w:
Case 1: If you are currently on 4.1, then you can restore to official 4.1 again, and when you see error 1015 pop-up, you kick your device out of recovery mode using TinyUmbrella, use redsn0w and don’t check “ipad BB update” because you already have that :)
Case 2: If you are currently on 4.2.1 then you’ll have to restore back to 4.1 (because 4.2.1 will not allow you 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 CASE-1.
If you 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 .
It is to remind you that iPhone Dev Team is also working on a new hack which will also allow users to downgrade the baseband from 06.15.00 to any previous baseband like 05.15.00.