The best logic I have heard over this is that this whole "leaked 4G" is being done to overshadow the release of all the Android phones coming out this quarter. Market wise, spring is a big time for cell phone sales for whatever reason. I could be a "leak" that is intentional to give those thinking to drop iPhone and move over to Android consideration of what is next.
My prediction is that the "real" next generation phone is going to have elements of the "leaked" phone. That is, forward camera, noise cancellation microphone, camera flash, HD screen and micro card. However, the industrial design is going to be totally different from what was saw.
I believe the phone that was leaked could be what is called a "hardware feature" unit that is used by developers to get the code working. As that is happening, the turtleneck, ponytail and emo glasses crowd spins out gobs of different industrial design form factors to try to get the curves and tactile response just right. Then these hardware features are jammed into the final form factor, a turf war happens for how big of a battery fits in vs. "user experience' and it comes out to the pubilc.
FWIW, I think you will see a next generation iPhone that is very close to the current iPhone 3G / 3GS style with millimeter here and there give and take. The flash, forward camera, noise cancellation microphone and HD screen will be in it running iPhone OS 4.