We've seen Raspberry Pi-powered tablets before, but they're always a bit clunky to actually build. Adafruit has put together a guide that shows one of the easiest, best looking, and cheapest Pi tablets we've seen.
You'll need a bit of soldering skills to pull this off, but otherwise it's a surprisingly simple project. You'll just need a Raspberry Pi, a 7-inch touch screen, and a few switches, cables, and other miscellany. The whole thing's packed inside a 3D printed case, which you can easily have made by one of these services. Sure, you can probably get a cheap Android tablet for the same price, but it's not nearly as cool as making your own.
7" Portable Multitouch Raspberry Pi Tablet [Adafruit]