It looks like someone was smart enough to bring their x-ray camera with them to the iPad hands on time. The iPad 2, much like the iPhone 4, is over half battery. Keeping these devices small and light seem to be limited by the power consumption requirements.
Here is the 1st generation iPad opened up.
It is worthy to not that the iPad 2 battery is likely thinner than the original iPad battery. With this assumption, the internal volume of the may be the same if the battery length was double...but the thickness was sliced in half.
Power Usage -> Battery Size -> iDevice Weight
This is a vicious circle...maybe Apple can develop a liquidmetal fuel cell for the next iPad to make it even lighter!
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