I have received lots of complaints both by email and in comments here regarding problems when unlocking iPhone 3G running firmware 3.1 with ultrasn0w. iPhone 3G users who have upgraded to 3.1 stock from iTunes will get this problem because of the 05.11.07 baseband, which makes it impossible to unlock it with ultrasn0w. The current version of ultrasn0w can only unlock iPhone 3G on firmware 3.1 with older baseband version i.e. 04.26.08. Fortunately, there is a way to downgrade your baseband to 04.26.08 on your iPhone 3G so that you can unlock it with ultrasn0w on firmware 3.1. Here’s how you do it:
Step 1: To downgrade your baseband from 05.11.07 to 04.26.08, you must first jailbreak your iPhone 3G running firmware 3.1 by following the step-by-step guide posted here.
Step 2: After you have jailbroken your iPhone, Go to “Cydia” on your iPhone and Search for “Fuzzyband” and then install this application.
Step 3: Start “Fuzzyband” app and touch on “Downgrade” button to downgrade your Baseband from 05.11.07 to 04.26.08.
Step 4: You should now be able to perfectly unlock your iPhone 3G using the latest version of ultrasn0w by following the step-by-step guide posted here.
NOTE: Downgrading of baseband will only work on phones that have the 05.08 (5.8) Bootloader from the factory. If you do not know your bootloader then you can run Fuzzyband and it will report the version number for you.
Feel free to ask me any questions here should you run into any problems when downgrading baseband on your iPhone.