Most of the unlockers who updated the baseband of iPhone 3GS / iPhone 3G to iPad baseband 06.15.00 in order to unlock the device on iOS 4.2.1 using Ultrasnow 2.1 can’t downgrade the firmware to any stock version above 4.1. They can downgrade their devices only to custom firmware cooked by PwnageTool or Snowbreeze.
We have got a good news for all those who are not comfortable with baseband 06.15.00.
MuscleNerd confirms over Twitter that iPhone Dev Team is working on a new method that will allow users to downgrade the iPad baseband 06.15.00 to lower versions like 05.15.04 on iPhone 3G(S).
@Lezanz sorry still collecting data on that. But also working on a bb downgrade (!) method for those with unlockable bb
@idansc yes if the BB downgrade method works, it will work for 06.15 too.
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