Fuzzyband has been upgraded to support the latest baseband 05.13.03 in iPhone OS 4.0 Beta. The following instructions will only help downgrade iPhone 3G which is currently on iPhone OS 4 Beta with 05.13.03 baseband and older bootloader 05.08. This wont work with iPhone 3GS.
If your iPhone meets the above criteria, you can then use the latest version of Fuzzyband which supports iPhone 4.0 Beta to downgrade your 05.13.03 baseband to the older 04.26.08.
Once you have downgraded your baseband, you can then use ultrasn0w to unlock your iPhone on iPhone 4.0 firmware. Here’s how you do it:
Step 1: To downgrade your baseband from 05.13.03 to 04.26.08, you must first jailbreak your iPhone 3G running firmware 4.0 Beta by following the step by step guide posted here.
Step 2: Once 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 05.12.01 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: For those of you who don’t know your bootloader, simply run Fuzzyband and it will report the version number for you.