It is now being rumored from the iPhone's OS 4 SDK that it will be capable of recording 720p high definition video. Data strings in the SDK showed a support for resolutions up to 1280X720p, which is the same resolution used for HD content.
We are pretty sure it will have an upgraded camera, up to 5 MP but, there is no real evidence that it will support recording HD video. Gizmodo received a prototype to play around with but, most of the hardware was not enabled to perform any real tests.
Apple is rumored to unveil the new phone some time in early to mid June.