Thanks again to the iPhone dev team there is already a solution for this, once you have jailbreaked your iPhone:

• make sure your iPhone indeed runs iPhone OS 3.0 and Cydia is already installed and working!
• open Cydia, the first time you open Cydia it does some cleaning and reorganisation, so if it closes after you opened it, just try again.
• add the repo repo666.ultrasn0w.com to Cydia.  That last “o” is actually the number zero “0”!
• search for ultrasn0w and install it
• reboot your iPhone
• T-Mobile USA users should disable 3G before using ultrasn0w
• enjoy