It was bound to happen, but it has. Someone's got the Wii U gamepad to work on the PC. You can do a bunch of mapping now but the touchscreen and streaming features aren't there yet. The developer says he's working on it and it should be coming in the near future, but you can grab the files from here.
Unlike the Wii controller which is just Bluetooth, the Wii U controller works via Wifi since it needs the bandwidth for things like streaming. So it's not as simple as connecting the controller as there's a server application that needs to run to interact with the controller. It will take a little tinkering such as running XAMPP, but I wouldn't be surprised if the process simplifies even more as future iterations are released.
Check out the video of it in action. Now, if I can only find just the controller itself to purchase to try this out.