Dev Team released a new redsn0w version which allows Apple users to downgrade the 06.15.00 baseband of the iPad which was installed on the iPhone 3G and iPhone 3GS smartphones.
The downgrade is done at 05.13.04 which can be unlocked using the most recent redsn0w version, however it will allow you to use the GPS of your devices without using a internet connection with the Maps application or other similar apps.
This solution works on any iOS version and it can be used only on the iPhone 3G and iPhone 3GS smartphones. You can’t downgrade the baseband of the iPhone 4 / iPhone 4S smartphones at the moment.
You can’t also downgrade from another baseband of the iPhone 3G / 3GS, hoever you can update to the 06.15.00 baseband of the iPad using this tutorial and you can then downgrade to 05.13.04 baseband.
What is important to remember is that the baseband of the iPad tablet does not work with iPhone 3GS smartphones released after the 24th week of the year 2011. Precisely, iPhone 3GS with a serial number including “xx134″ or bigger do not support the 06.15.00 baseband of the iPad tablet.
Download the most recent redsn0w version from here: http://blog.iphone-dev.org/post/25350690843/0615-fun
Make sure you have a proper IPSW.
Connect your Apple device to your PC or Mac. In Windows, to go the folder where you saved the new redsn0w version, right click on the executable file, select Properties and in the new menu go to Compatibility and check Run This Program in Compatibility mode after which select Windows XP, hit Apply and then OK.
If you don’t follow this step you won’t be able to start your device in tethered mode.
On Mac / Windows, hit the Extras button and then Select IPSW.
Close your iPhone using the Lock button. Press the Lock button for a few seconds until you see the red slider, drag it from the left to the right and your device will close. This step is important for the entire process, and if you skip it you don’t be able to use redsn0w to downgrade your baseband.
Put your iPhone in DFU Mode.
In Redsn0w, return to the main menu and choose the Jailbreak option.
In the new menu check Downgrade from iPad baseband and disable the option to install Cydia if you jailbreak over an existing jailbreak solution. If your device is not jailbroken then you can also select the Cydia installation option.
The baseband downgrade process will take a pretty long time, and the Flashing Baseband procedure will take between 3 to 8 minutes so you have to be patient. After the process is done and your device restarts then you should have a signal, if ultrasn0w was already installed and in the Settings > General > Modern Firmware you should see the 05.13.00 value.
This baseband can be updated through a normal update process through iTunes, so you should only use custom ipsw’s to keep the initial value, otherwise you have to update again to 06.15.00 and then downgrade, again, to 05.13.00.