1-Use the redsn0w tool to downgrade the baseband to 05.13....
2-If you require the iPhone to be unlocked you will need to preserve the basband before upgrading
3-Use iTunes to update firmware to 5.1.1
4-You will need a sim card for the network the iPhone was originally on, to Activate the iPhone (at the end of the update to 5.1.1)
5-Jailbreak iPhone....and assuming your have taken steps to preserve baseband you can go ahead and unlock iPhone
Here's a little more info on the "Downgrading from 6.15.00 to 5.13.04" part. http://www.iphoneforums.net/forum/iphone-unlocking-69/downgrading-3g-3gs-baseband-05-13-04-a-42452/ The reason you need to do this... because while you're on the 6.15.00 iPad Baseband you're always requred to use a Custom IPSW (Firmware) file to update with. This downgrade process to 5.13.04 allows you to use ultrasn0w for your Unlock and for you to use standard IPSW's for future updates.