pH (Pedro Henrique Franceschi) at BigBoss has released pHaseBanDowngrader, which will downgrade your iPhone baseband from 02.30.03 to 02.28.00.
Why on Earth would you want to do that, you might ask?
Well, same reason some people rip Vista off that new PC and install Windows XP in its place. Just as there are some Windows programs that don't run worth a rat's ass under Vista, there are jailbreaking apps like YellowSn0w that won't work under the newer baseband — usually as a result of upgrading the iPhone OS to 2.2.1.
[Get the pHaseBanDowngrader code and full instructions at BigBoss]