Update : MuscleNerd will release downgrade 6.15.00 and fix GPS problem after iOS 5 public release - Click Here.
Here's a bit of good news for who suffered from the new unlockable baseband 6.15.00 on iPhone 3GS and 3G, most of the unlockers who unlocked their devices with ultrasn0w 1.2 can't restore to any stock firmware above 4.1, it only can be restored to a custom firmware, moreover, ultrasn0w 1.2 caused a loss of GPS. Let's check out the new progress regarding downgrade 6.15.00 baseband.
Lezanz: what about GPS issue? Any progress?MuscleNerd: has just confirmed that he is working on a new method to downgrade baseband 6.15.00 to lower versions, which will be unlockable and to fix the GPS issue. Till now we didn't catch more details, but of course will let you know once we catch more, stay tuned.
Musclenerd: sorry still collecting data on that. But also working on a bb downgrade (!) method for those with unlockable bb