A patent was just filed about a week ago detailing what's inside the Wii U Tablet controller.
http://www.freepatentsonline.com/20110190052.pdf
The tablet has (in addition to what we knew before), a magnetometer, an IR port, a speaker, a microphone and built in flash memory!
Pictures from the patent:
It also seems to have a built in rechargeable Lion battery (rather than using AAs)