Repair Labs compared the shell to an iPad 2 case, a noted a number of differences between the old and new.
a) The logic board mounts were differently, implying the board will be shaped differently.
b) In addition the iPad 3 will have a larger battery pack, based on the logic board width.
c) The iPad 3 will also have a different camera, although they couldn't give any specifics based on the rear shell. However, the rear --- and front --- cameras on the iPad 2 are admittedly quite anemic compared to those of other tablets.
d) There has been much speculation about the iPad 3 shipping with a "retina display" and although the site can't make out details on that based on the rear shell, the mounts, they said, indicate that the LCD will, at the very least, be redesigned.
Earlier rumors were that the iPad 3 would be slightly thicker than the iPad 2. That could be to include a larger battery, or to include LTE support, or both. However, Repair Labs' source said that it was possible that the thickness rumor would turn out to be false.
One other thing coming from a different source. Although Android tablets have already been produced with quad-core processors, it appears the iPad 3 will stick with dual core.
Instead, the iPad 3 will sport a significantly improved GPU, all the better to power the 2,048 x 1,536 retina display. Assuming Apple sticks to its annual refresh cycle, we should find out soon the truth of the matter.
Apple's been fairly consistent on releases of its iDevices, but the iPhone 4S was months later than its normal summer refresh.