You can now downgrade your iPhone 3G baseband from 02.30.03 back to 02.28.00. Users who upgraded to firmware version 2.2.1 accidentally without following the dev team instructions were left out of the yellowsn0w tool.
For those who have wanted to unlock their iPhone 3G but have updated to the 2.30 baseband, hope is now here. Thanks to the work of dev pH, a tool has been created to downgrade your baseband, and allow a unlock to occur once again.
So if,you have firmware v2.21 with baseband 2.30 and bootloader 5.8 you can go back to the old baseband using the modmyi tool here.