The SDK release number is 9m2192 and the iPhone firmware release number is 5a308.
According to TUAW, "App Ids are a new requirement in provisioning profiles for iPhone OS beta 6, to continue developement with the iPhone SDK, developers must create application IDs and regenerate all provisioning profiles. Previously generated profiles will not work with SDK beta 6."
To use the latest SDK you must be running the newly released Mac OS X 10.5.3.
You can view the release notes here