Tomorrow Dev Team will release a baseband downgrade method for iPhone 3G and iPhone 3GS

  Maine Dev Team is about to release a solution by which iPhone 3G/iPhone 3GS terminal owners will be able to downgrade the 06.15.00 baseband of the iPad tablet to the 05.13.x baseband which is decodable via ultrasn0w. The solution will be implemented in a new version of redsn0w which will most likely require users to make a custom ipsw through which the baseband will be downgraded. To do the whole procedure you must first save everything you have in the devices and only then install that baseband and I think everything will be available for iOS 4.x – 5.1.1.

  After you actually downgrade the baseband, you will be able to decode your terminals with ultrasn0w, but you will be able to use the GPS that this baseband blocks. After performing the procedure, you will be able to use without problems any navigation applications with the GPS included in your iDevices and you will no longer be conditioned by the activation of an Internet connection. This solution will only work on iPhone 3G/3GS that have the iPad baseband installed, I don't know if it will downgrade the other basebands of these devices, but it is clear that you will not be able to downgrade the basebands of the iPhone 4 terminal .

  I will give you more information tomorrow, as soon as the Dev Team publishes the new downgrade solution.