The Wii U will be region locked. Nintendo confirmed the restriction to Eurogamer today after the initial revelation in Japanese game magazine Famitsu.
Region locking means that any given Wii U console will only be able to run games from its own intended geographical area; no playing Japanese or European Wii U games in North American consoles and vice versa (at least without modifications to the system).
The limitation is par for the course for Nintendo, which put the same restrictions on the Wii and GameCube. Both PlayStation 3 and Xbox 360 allow publishers to decide whether to lock certain games to any regions or not.