Finally after long wait iphone dev team has released the only unlock tool for iPhone 3G 3.0 firmware with baseband 04.26.08, Ultrasn0w. There has been delay of about three days in releasing ultrasn0w due to unexpected bugs.
You must first jailbreak your iPhone 3G 3.0 with Redsn0w 0.7.2 and then add cydia installer as Ultrasn0w is available via iPhone app repository only.
Ssteps to Install Ultrasn0w & Unlock iPhone 3G 3.0 Baseband 04.26.08
1. Please add the repo repo666.ultrasn0w.com to cydia or icy
2. Search for ‘ultrasn0w’ in cydia or icy and install ultrasn0w
3. Reboot your iPhone 3G
4. T-Mobile USA users should disable 3G before using ultrasn0w
T-mobile USA users dont forget to disable 3G before using Ultrasn0w
How to Disable 3G Mode in iPhone 3G
Go to your iPhone 3G Settings->General->Network and turn off 3G Mode.