Just as the new iPad has a much denser battery than previous models, I'm sure the same technology would be employed in any new notebook line. Plus there may be some space left for a larger battery with the removal of the optical drive, even after taking into account the rumoured shrinkage of the form factor.
Except the new iPad does not have a 'denser' battery, it has a much larger and heavier battery, along with a graphic card that consumes mega power and generates lots of heat.