Update : MuscleNerd will release downgrade 6.15.00 and fix GPS problem after iOS 5 public release - Click Here.
After MuscleNerd has declared about the new amazing Redsn0w jailbreak features, he has been asked about the downgrade for 6.15.00 baseband on iPhone 3GS and 3G after using ultrasn0w, he points that the work on the downgrade issue won't start only after the release of iPhone 4 baseband 2.10.04 unlock which expected to see the light in less than a month.
Fammbizz asking MuscleNerd: when is ur bb downgrade comin out for 6.15.00Many of users who unlock their devices using ultrasn0w 1.2 noted problems with their GPS, so we expect that downgrade will fix this bug, moreover, there is speculation about the possibility to downgrade the baseband to unlockable one which will be compatible with the old version of ultrasn0w. Stay tuned
MuscleNerd replying: TBH (to be honest), that won't be looked into until after i4 unlock