it'd need to be no more than 1cm thick with an 8"+ screen (preferably at greater than 480p resolution), and it'd need to have at least 32gb of space, 5hr hr active battery (non-wifi), 10hr+ screenless-battery, play h.264 and divx movies, and it'd need a full-featured browser that includes Flash support (Hulu, in other words). I'd also want bluetooth, Keyboard pairing (via bluetooth would be cool) and word processing applications.
GImme all three of those and I'd easily drop 299-399 on it. I'd pay 50 bucks extra if it had cordless charging, too. I'd pay another 50 dollars if it had docking (And I'd drop another 100 on a docking station) so that I could literally use it as a pick-up-and-go PC/Mobility crossover. drop it in my station at home with a 24" monitor and full setup, and then pick it up and go for when I'm on the move. And, bluetooth folding boards for when I'm at a coffee shop.
It's ambitious, but give me taht and you've literally got the device of the century.