When the decoding solution appeared, I warned you that there could be some problems if you update the iPad baseband. Those of you who needed decoding updated and probably had/had problems with the 1015 error that appears after any normal restore from iTunes. If you restore from iTunes with an iOS 4.2.1 firmware downloaded from Apple, you will receive error 1015 and there are extremely high chances that you will not be able to remove the terminal from Recovery Mode with any program. If you end up in a situation where neither iRecovery nor Recovboot nor TinyUmbrella take the phone out of Recovery Mode, then the only solution is a downgrade to a previous version of iOS and update back to iOS 4.2.1 using a custom ipsw with iPad baseband.
iOS 4.2.1 has a somewhat more aggressive protection system and the programs that until now took phones out of Recovery Mode no longer work. iOS 4.1 and other older versions do not have such a system, so they are safe. So, if you have iOS 4.2.1 installed, you received error 1015 and you have no way to remove it from Recovery Mode, then the only solution is to downgrade. You can downgrade using the tutorial this si this. For iPhone 3G it is not necessary to use SHSH if you restore to iOS 3.x, for iPhone 3GS it is necessary to restore regardless of the iOS version to which you restore.
If you don't have SHSH for old firmware versions, you could try to install 4.1 on your phone, but if you get error 3xxx or "This iPhone is ineligible..." then I'm sorry but there's not much you can do, you have to wait for a program to appear which will take the terminal out of Recovery mode.
If you manage to downgrade, jailbreak, then a custom ipsw for 4.2.1 cu this tutorial. A custom ipsw 4.2.1 with iPad baseband made with PwnageTool is the ONLY method to restore to iOS 4.x without error 1015.
Nothing I said above applies to iPhone 2G/4 or iPod Touch/iPad.
