The Dev Team just posted on their site:
Ultrasn0w for iPhone 3G is ready!
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
If you relied on yellowsn0w to use your iPhone 3G you’re now able to upgrade to OS 3.0 and use Ultrasn0w to unlock your device!
[Source - Dev Team]