This is an incredibly clever solution for optimizing sites for readability for the iPad. I think it extends to more devices than just the iPad — MacBook Airs come to mind.
I would love to implement something like this on this site, because even when I am not viewing this on the iPad I sometimes want a different font size. If you know how to roll this into WP let me know.
(Note: I am against all mobile themes, but not against good optimization for readability. I want better readability no matter what the device being used is.)