iPhone dev team has released new version of Ultrasnow / Ultrasn0w today. Below reading my guide, please understand that this is not a new unlock for iPhones with new basebands. It merely fixes the signal bar issues on the existing devices. Also, it is meant for those iPhone which are already jailbroken with these baseabands 01.59.00, 04.26.08, 05.11.07, 05.12.01, 05.13.04, 06.15.00 and are running 4.3.1 using either Snowbreeze or PwnageTool.
First, you will have to jailbreak your iPhone using iPhone 4.3.1 Snowbreeze or iPhone 4.3.1 PwnageTool. Make sure to select the option "preserve old baseband" when using either of the options. Done? Then fire up Cydia on your iPhone. Go to Manage tag and tap Sources.
Tap Edit and then on Add. Enter http://repo666.ultrasn0w.com and tap on Add Source. Cydia will then refresh the repositories. Then go the "Search" tab and search for "ultrasn0w 1.2.1" (note there is a "0", not "o"). Install it and then you will be able to use your iPhone 4 with any carrier.
Make sure to restart your iPhone.