Since all recent iPhone 3GS basebands (i.e. 05.14.02 and anything newer) no longer have the AT+XAPP Vulnerability, the iPhone Dev Team has released an update to its jailbreak tools (redsn0w 0.9.6b5 and PwnageTool 4.1.3) that allows one to install this iPad version which still is vulnerable. This works because part of Apple's security system is to only allow the installation of signed baseband firmware with a higher version number. Since this firmware has a higher version number, it can be installed even though it is older. The benefit is that it allows for unlocking the phone. The downside is that it breaks GPS functionality, however it can be downgraded again to 05.13.04, which will restore GPS functionality and still have the vulnerability.
The latest tool that is capable of installing iPad baseband is redsn0w, for all firmwares since iOS 4.1. This baseband works fine up to iOS 4.3.5 (except for GPS functionality) and on iOS 5.x it requires a fix, which is implemented in redsn0w 0.9.9b6+. Initially, the baseband could only be downgraded on an iPhone 3G with Baseband Bootloader 5.8, or by a baseband chip replacement. As mentioned above, a method has since been discovered that lets iPhone 3G and 3GS owners downgrade their baseband to 05.13.04; it is available in redsn0w. The only reason users should upgrade from 05.xx.xx to 06.15.00 is for unlocking and the baseband is higher than 05.13.04. The baseband 04.26.08 has more issues than 06.15.00 on iOS 3.1+, but ultrasn0w can fix them up to iOS 5.0.1. If you have xx128 or higher, you should not install iPad baseband as it will brick your device.