Apple has posted iPhone OS 3.0 beta 2 (build 7A259g) in the iPhone Dev Center. The download is only available to registered iPhone developers and weighs in at 2.13GB.
According to Ars Technica the second public developer build of iPhone 3.0 contains minor updates, bug fixes and no new functionality. Sources tell Ars that Apple is warning developers to avoid its tethering and MMS features because they "are not supported, and should not be used."
As with the the previous developer build (7A238j) Apple warns:
Devices updated to iPhone 3.0 beta can not be restored to earlier versions of iPhone OS. Devices will be able to upgrade to future beta releases and the final iPhone OS 3.0 software.
Also it's wise to avoid the temptation to install an unlicensed version of iPhone 3.0 as it has been known to turn your iPhone into an iBrick.
Tip: Ars Technica