Restoring original baseband to iPhone 3G 4.2.1 with iPad baseband
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)).