Preview at WWDC likely in June, followed by September launch.
Hands On with a Physical Mockup of the iPhone 5
The Verge has since tracked down the same mockup and was able to take high quality photos and video of the device which show it well compared to the iPhone 4S.
The video does a good job showing off the size and heft of the device as compared to the current iPhone 4S. As noted in the video, this is a non-functional mockup. It's only used as dummy shell for case manufacturers. According to the case manufacturer, they are "very confident" about the new design and have started production of cases in volume in anticipation of the new iPhone.
The rumors for the design of the new iPhone have been very consistent. Apple is widely expected to announce the new iPhone on September 12th.