Yesterday Apple released iPhone Firmware 3.0.1 . This update is NOT a major update, and it only ads a patch for the SMS security flaw , which was explained by Miller at the Black Hat Conference a few days ago.
The firmware upgrade does not update the baseband and you can still use the same tools to jailbreak and unlock your devices after upgrade. But there is only one trick to the process. You can read how to jailbreak and unlock iPhone firmware 3.0.1 on Mac and Windows , here.
In case you are using firmware 3.1 beta 3, you already using a firmware that includes the patch for SMS you are already in a situation of complete security. Therefore, there is no need to perform a downgrade to version 3.0.1 .