WiiU OS uses Ubuntu (in demo kiosk) and has Open Source software.

[superchunk]

ebw said:

The download just contains some source code files for the Browser and XML parser.  Looks like they made some modifications to support the GamePad (e.g. inputs and video playback), the CPU, etc.  As for the XML library I am not sure what they changed, but that is such a generic function that it couldn't be a major difference.

I wonder whether the Wii U is really running Ubuntu or could it be just the kiosk?

If that Open Source code is just in reference to the browser, then yeah, its very likely it was just the demo kiosk and not the WiiU itself.