UltraSn0w Fixer For 5.1.1 Released To Unlock iOS 5.1.1 On iPhone 4, iPhone 3GS for more information follow this
How to upgrade a previously jailbroken iPhone to iOS 5.1.1 and preserve the baseband.
The process of getting this job done requires a number of steps (but are easily done), which are downloading the official iOS firmware from Apple, upgrading iTunes to the latest version, getting the latest redsn0w from the dev team, making your custom iOS, restoring your iPhone to the custom iOS, restoring your iPhone data, installing Cydia and lastly installing Ultrasn0w. If you are new to this it can well be intimidating with loads of jargon but take your time and go through the information below.
This process has been tested and successfully installed/Jailbroken on a iPhone 3GS old boot rom device that has been previously jailbroken and had the iPad baseband firmware of 6.15 applied.
If you have previously jailbroken your iPhone, the only way to get to the latest iOS 5.1.1 is from a custom iOS IPSW which is made from a stock Apple standard iOS IPSW. In which case you will want to follow the whole post.
If you are on an unjailbroken iPhone stock iOS 5.1.1 and just want to jailbreak go to the second half of the post.
Mac/Win users can create their own custom IPSW with the coolest software app from the dev team called ‘redsn0w’.
Please note that this is only an untetheredjailbreak for iPhone old-boot rom 3GS, but atethered jailbreak for new-boot rom 3GS Phone 4, iPhone 4-CDMA, iPad1, iPodTouch 3G, iPodTouch 4G. Untethered meaning that you can reboot the device normally without software assistance. click
This will not work on iPhone 4s and iPad2/iPad2 known as A5 devices.