How do I downgrade my iPhone 3.1 baseband to 04.26.08?
Updating to iPhone firmware 3.1 using iTunes will also update your baseband from 04.26.08 to 05.11.07. To unlock your iPhone for use on a different GSM carrier with ultrasn0w this modem firmware must be downgraded to the older version.
Unfortunately, downgrading the baseband using the tools below requires a jailbroken iPhone 3G. The solution will not work on an iPhone 3GS or an original iPhone.