iPhone 3.1 Beta 3 is now accessible to those with an existing developer account. Apple also released a new iPhone SDK for OS 3.1.

iPhone OS 3.1 adds Bluetooth and video features. Users can now invoke Voice Control using a Bluetooth headset rather than a wired headset or the built-in microphone. When editing video clips, users can save a copy of the trimmed video instead of permanently losing the discarded ends. The new SDK also allows developers to connect to and work with a system wirelessly, without the tether of a USB cable.

The new release may be ready as son as early September, just in time for Apple’s iPod event — and the possible release of an iPod touch with a camera.

The update will reportedly allow for augmented reality apps, which overlay information and controls on top of real-world objects seen through a camera: Acrossair was told by Apple that the 3.1 release would be required for its Nearest Tube and future Nearest Subway apps to work properly.

These class of apps are highly dependent on the built-in compass and autofocusing camera of the iPhone 3GS, both of which are required to recognize the direction the iPhone is facing and to get a detailed enough image of the subject to tag it with information. As a demonstration of the technology, Acrossair’s software can show the subway stops visible in a particular direction and their distance relative to the user (see video).