Looks like the iPhone Dev-Team's Ultrasn0w unlock for legacy iPhone 3G owners is ready. The unlock works with the original iPhone as well but not the iPhone 3GS - for that you'll have to be patient.

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. That last �o� is actually the number zero �0�! If you use the letter �o� you'll get an error.
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