UPDATE: The iPad 2 thickness is 8.8mm
The next iPad will definitely not be thicker than the original based on previous Apple innovations, but there is definitely room for improvement. If BlackBerry can make their Playbook 25% thinner than the iPad, and include a USB port....Apple has room to slim down a little bit. My MacBook Air is 1/4" thicker with 128GB of space.
I expect the iPad to be close to the iPhone 4 thickness of 0.37" and closer to 1 pound. This is definitely possible with the switch to a carbon fiber exterior instead of their signature brushed aluminum unibody.