Preview at WWDC likely in June, followed by September launch.
Leaked Photos of the 'iPhone 5' Logic Board
Nowhereelse notes that the screw holes of the logic board lines up exactly with those of the previously leaked shells The site even created this animated GIF which shows how the holes line up. The image appears to be mirrored, presumably for clarity.
We've seen a larger number of part leaks for the next generation iPhone, but this is not a new phenomenon. Last year, we saw many parts for what would become the iPhone 4S, including its logic board revealing an A5 processor. This leak is somewhat less revealing as much of the shielding remains in place.
The increased number of part leaks likely corresponds to the belief that Apple is already ramping up production of the next generation iPhone. Apple is expected to launch the new iPhone on September 12th.
Update: The same forum poster posted early photos of the iPhone 4S logic boards last year.