WARNING: DO NOT install the iPad baseband on the 3GS. The newer 3GS (week 36 & above of the year 2011 (Serial No:xx136xxxxx)) will DEFINITELY brick your device during 06.15 baseband flash. It has been confirmed to permanently damage the built-in radio hardware, thus rendering it useless pretty much forever. It doesn't matter whether you integrate it into a custom IPSW or use Redsn0w. As of now the only solution is to return to Apple store & get it replaced. The iPad BB unlock solution had its piece of happy times & now its over. So think twice. The older iP3GS's (week 35 & below) are safe. A more detailed discussion about this can be found at JailbreakQA.
MuscleNerd, who happens to be one of the core members of Dev-Team just confirmed that downgrade of iPad baseband 06.15.00 to 05.13.04 is very much possible and is 99% doable. Downgrading iPad baseband back to 05.13.04 will not only fix battery drainage issues but will also help iPhone 3GS/3G users to fix their broken GPS.
@Kamiela2010 the downgrade potential from 06.15 remains (in my mind) 99% doable, but other things always seem to preempt working on it
In reply to:
@MuscleNerd it seems you guys have no interest in fixing gps issues with 6.15.00. Am I right??
Back in November 2010, Dev-Team released Ultrasn0w 1.2 to unlock iPhone 3GS (Baseband: 04.26.98, 05.11.07, 05.12.01, 05.13.04, 06.15.00) and iPhone 4 (Baseband: 01.59.00) on iOS 4.1 & iOS 4.2.1. Those who were running 05.14.02 or 05.15.04 baseband on their iPhone 3GS/3G were given with an option to upgrade their baseband to iPad baseband 06.15.00 if they want to use Ultrasn0w 1.2 to unlock their iPhone. Upgrading to iPad baseband not only made some iPhone 3GS users to lose their GPS but they also faced severe battery drainage issues.
Upgrading your iPhone 3GS to iOS 4.3 upgrades its baseband to 05.16.01 and restoring stock iOS 4.3.1 upgrades its baseband to 05.16.02. As both of these basebands are < than iPad baseband, so practically it is possible to upgrade them to 06.15.00. Now if you are thinking about upgrading your iPhone 4 (02.10.04, 03.10.01 or 04.10.01) baseband to iPad baseband then don't be too smart because iPhone 3GS/3G basebands do not require SHSH blobs to flash iPad baseband but the iPhone 4 does :P
According to MuscleNerd, instead of working on the unlock of individual baseband (05.14, 05.15 or 05.16), he will devise a single downgrade method for all iPhone 3GS users running iPad baseband. In this way, those iPhone 3GS/3G users who have baseband 05.14.xx, 05.15.xx or 05.16.xx will be able to use Ultrasn0w 1.2 by first upgrading to iPad baseband and then downgrading back to the unlockable baseband 05.13.04.
Conclusion: If you are waiting for the unlock of iPhone 3GS 05.14, 05.15 or 05.16 baseband then forget about it. In either way you MUST first upgrade to iPad baseband and then downgrade back to baseband 05.13.04. The best method to upgrade your iPhone 3GS to iPad baseband is by using Sn0wbreeze 2.5.1. Just download Sn0wbreeze 2.5.1 from here and follow this guide to upgrade your baseband to iPad baseband. If you are on Mac, use Redsn0w 0.9.6b5 instead of using Sn0wbreeze.
Update#1: Fix iPhone 3GS (iPad Baseband) Battery Life Issues on iOS 4.3.3 [Tips]Contact Us for News Tips, Corrections and Feedback