Update it has been released: True GPS and Baseband Downgrade Now available via Redsn0w:
Get ready, click here
Musclenerd has successfully been able to downgrade a 3GS to baseband 05.13.04, it is also possible to downgrade to any baseband! This means 3GS users on Baseband 06.15.00 will have their GPS functionality restored.
The most encouraging tweet from MuscleNerd has been this one:
I am aiming for Funday/Sunday for releasing the downgrade from 06.15. but that's very much subject to possible delay.
Hold on guys, stay subscribed. Will make a video and tutorial as soon as it is released, along with downloads :)