The dev team has released Ultrasn0w their latest carrier unlock for firmware 3.0. unfortunately, it doesn’t yet works for iPhone 3G S, but it’s safe to upgrade to OS 3.0.

1. Ensure you have upgraded to iPhone OS 3.0
2. Jailbreak your iPhone 3G using redsn0w or PwnageTool (this will also install Cydia/Icy)
3. Run Cydia or Icy
4. Please add the repo repo666.ultrasn0w.com to Cydia or icy
5. Search for ‘ultrasn0w’ in cydia or icy and install ultrasn0w
6. Reboot your iPhone 3G
7. T-Mobile USA users should disable 3G before using ultrasn0w
8. Enjoy

i already followed their step have jailbroken(using redsn0w) and unlocked(ultrasn0w) my iPhone 3G,works flawlessly. Thanks Dev Team!!!!