A bunch of hackers have managed to get the original Wii's Homebrew Channel working on the new Wii U.

"We have a new release with a new version of The Homebrew Channel; this will install on both Wii consoles, and inside the virtual Wii sandbox inside the Wii U. The currently used IOS exploit is courtesy of tueidj.

"Please note that this is not running in full Wii U mode; running this on a Wii U will probably work just like on a Wii. Also, due to technical limitations of the virtual Wii mode, BootMii will not work on a Wii U. If we are able to resolve this, we will make a new release with BootMii support."
Source: HackMii