Apple has seeded the latest Beta 3 version of iPhone OS 3.1 Build 7C116a along with a newer version of iPhone SDK 3.1 Beta 3 Build 9M2808 to iPhone Dev Center. Members enrolled in the iPhone Developer Standard or Enterprise Program can access the development bits for iPhone OS 3.1 Beta 3 by signing into Apple Developer Connection.
It remains to be seen whether or not this latest build will bring the support for augmented reality apps, rumored to be a part of the upcoming 3.1 firmware.
NOTE: Those of you who have jailbroken and unlocked their iPhone with OS 3.0 should avoid to upgrade to 3.1 unless new tools are made available by Dev-Team (this warning does not apply to iPhone 2G).
Download iPhone 3.1 Firmware Beta 3 (Apple Developer Connection)