This article has been updated to reflect that Wii U Hax is a news website that has tested the work of others, as opposed to being a "hacking group", with two additional amendments in square brackets.
Every piece of gaming tech is potentially hackable, but so far the Wii U has proven to be a tough nut to crack. There have been attempts of course, but according to Nintendo these have come to nothing.
It would now appear that the situation has dramatically changed. A news site known as "Wii U Hax" has revealed that it has tested an exploit to access the Wii U hardware — through the Wii mode, at least — and is proudly claiming that this is [a potential] beginning of the Wii U homebrew scene.
It's not known if the hack grants full access to the hardware, but Wii U Hax believes that the exploit can't be patched by Nintendo without releasing a new hardware model of the console.
System integrity has always been a key aim for Nintendo — especially after the piracy-packed days of the DS and Wii — and the company has clearly made things harder with the 3DS and Wii U. System updates can be effectively employed to block any potential weaknesses, but if what this [source article] is saying is correct, this is one hole that may be impossible to plug.