According to German hardware site 01net.com, Wii U hardware development has been rushed off to the extent that they haven't managed to produce a working prototype yet.
01net is one of the most trusted sources for leaked info about hardware development and their sources have described the Wii U current state as a "development hell."
According to said sources, the main chipset of the Wii U controller is inadequate due to Nintendo's too much cost cutting and too short development schedule. Nintendo has already created three different prototypes till now; none of them has its wireless functionality working properly.
A fourth prototype is expected by the end of the month.
As it stands, developers are forced to use a tethered controller to develop and test their games. Each controller is fitted with a small black box that handles the connection, but that comes with another set of problems and it doesn't work properly.
There is no official release date for the Wii U yet but a lot of developers expect it to be around June 2012, less than 10 months from now. If 01net's sources are correct, then there is hardly enough time for Nintendo's engineers to finalize the design and commence with the production of a few million units before launch.