Miratech has just released some data from a recent study. The study compared how a group of iPad owners read on the iPad versus how they read the same article on a physical newspaper.
The study group spent about the same amount of time reading each item, but they remembered more and better details about the story in the paper.
Miratch also tracked the eye movement of the participants and careful analysis revealed that the eyes lingered longer on the paper version. Here are a couple videos that demonstrate the difference in eye movements.