Restoring original baseband to iPhone 3G 4.2.1 with iPad baseband A friend of mine bought an iPhone 3G from some T-Mobile cell phone vendor. The vendor had installed a custom 4.2.1 ipsw with the iPad baseband because iPhone 3G's wasn't unlockable at the time. That was great at first, but then my friend tried to restore the iPhone through iTunes. Because of the baseband, in the middle of the restore (the bar on the iPhone was partially filled) iTunes would vomit an error and axe the whole process, essentially bricking the device. He usually takes his computer issues to me, so I managed to fix the issue by reading forums and by finding an ipsw online with iPad's baseband. 

That iPhone's been working for a while now, but if this happens again, is there a way I can revert from the iPad baseband so he can restore through iTunes without my intervention?
You cannot restore the baseband. Upgrading to iPad baseband 6.15.00 is permanent (unless for some magical reason Apple wanted to come out with new firmware for the iPhone 3G/3GS that miraculously contained a baseband higher than 6.15.00). What you have to do is use redsn0w to remove the baseband from an .ipsw file (the iOS firmware from Apple) if you want to upgrade/restore. There's an excellent tutorial on how to do that here. Note that you it will always need to be JailBroken, too (which that tutorial has you do), because iTunes will not verify your .ipsw under these circumstances (even if you try to load on the modified firmware without JailBreaking and then taking it into iTunes to complete the verification/activation (believe me, I've tried)).