Typically you'd have the 06.15 baseband if you unlock with ultrasn0w but updated your iPhone baseband past 05.13.04. With this new capability, you can now downgrade specifically from 06.15 to 05.13.04 (even if you never had 05.13.04 on that device before). This gives you the best of both worlds: ultrasn0w compatibility and a normal iPhone baseband with full GPS and the ability to use stock IPSWs again.
1. Use the "Extras->Select IPSW" button in redsn0w to tell it which firmware version you have installed (new-bootrom 3GS users can usually skip this step but it doesn't hurt for them to do it too).
2. Do a controlled shutdown of your iPhone ("slide to power off"). This step is very important to avoid mount problems when the ramdisk is running!
3. Go back to the first screen and click "Jailbreak". Enable the "Downgrade from iPad baseband" checkbox, disable Cydia if you already have it installed, and click Next proceed through the normal DFU ramdisk steps.
Note: The "Flashing Baseband" step TAKES A VERY LONG TIME to complete (3-8 minutes).
For those who are wondering if you can update your 3G or 3GS to 06.15 solely for the purposes of downgrading to 05.13.04, the answer is "yes" for 3G owners, and "maybe" for 3GS owners. The iPad baseband is not compatible with 3GS units manufactured week 34 of 2011 or later. If you have an iPhone3GS and if digits 3-5 of its Serial Number are 134 or later (xx134 ), then you should NOT try to install the 06.15 baseband on your 3GS! It will brick your radio, preventing both the downgrade from working and normal iPhone software from using it as a phone! Be warned!
RedSn0w will be updated to offer the new feature within the hour. You can follow iClarified on Twitter, Facebook, or RSS to be notified of release.
RedSn0w 0.9.14b1 has now been released. You can download it from here. Tutorials to follow shortly.