The new baseband 06.15.00 used to unlock iphone 3GS and 3G can be downgraded to 05.15.04 according to Iphone Dev Team member Muscle Nerd . He also confirmed the Un-Tethered Jailbreak for iOS 4.2.1 on iPhone 4 scheduled to be released on or before christmas this month.
Baseband upgrade on Iphone 3GS 3G was done to exploit the vulnerability still left in iPad firmware baseband 06.15.00 and was used to unlock the phones using UltraSn0w 1.2 Unlock on iOS 4.2.1 .
However the unlock came with several problems like Battery Drainage and GPS issues .
Above conversation between Muscle Nerd and iPhone users suggest that the developers are working to downgrade the baseband on the upgraded phones .
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