The Wii U has its first demo via Nintendo's eShop in North America and it is the incredibly popularFIFA Soccer 13. It may be a bit difficult to find, since the shop has no dedicated area for demos, but it's there.
Joystiq was able to find the demo and confirm it was the only one available now. It weighs in at 1.2GB and can be found on the store's page for the game. It was a launch title for the console and has received only average review scores.