Above is a leaked photo of what is believed to be the case back for an iPad 3. On the left is the case back for an iPad 2 and on the right is the leaked photo for case back of an iPad 3. Comparing them side-by-side allows us to make some guesses about what the iPad 3 will be like.
The new case indicates that the iPad 3 will have a larger battery, slightly reconfigured logic board, new cameras, and a different type of screen. However you may want to take the leaked photo above with a certain level of skepticism because Apple is very good at keeping secrets when it comes to upcoming products.
Other sources indicate that the new iPad will have a double resolution display with a true iPad Retina Display (2048 x 1536). The device size will be physically identical to the iPad 2, but it will be about 1 mm thicker overall.
There will be an A6 CPU in the iPad 3, but it will not be quad-core. It is expected to be a faster version dual-core CPU.