You Will Soon Be Able to Downgrade Baseband 06.15.00 to 05.15.04 on iPhone 3GS / 3G
Earlier today, MuscleNerd confirmed that he has got a “backup” method of making jailbreak untethered on iOS 4.2.1. This new method utilizes iOS 4.2b3 (Beta 3) SHSH blobs which Cydia only recently started saving for all iOS devices.
And now, in a reply to Twitter users Lezanz and idansc, MuscleNerd also confirmed that the IPhone Dev Team is working on a method to downgrade06.15.00baseband back to 05.15.xx so that iPhone 3GS and 3G users who upgraded to iPad’s 06.15 for iOS 4.1 / 4.2.1 unlock can restore to the stock iOS firmware.
@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.
We will of course let you know as soon as we have more on this! Stay tuned ! Source: Twitter , 
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