Nintendo's Wii U is the follow-up to the Wii, and, like the Wii, it is meant to be played indoors. The Wii U's most prominent feature is the six-inch touchscreen embedded in its controller, which makes it easy to see why it might be mistaken for a portable game system. However, the Wii U controller wirelessly beams information to and from the Wii U console. Even though you can play Wii U games on the controller's embedded screen instead of on a television, the controller is not a separate game console. You can take a Nintendo DS or 3DS anywhere; if you try to do the same with the Wii U controller, it will not work.
At Nintendo's E3 2011 press conference, President Satoru Iwata made it clear that the Wii U is "not designed to be a portable gaming system." If you want on-the-road gaming goodness, you'll have to grab a Nintendo DS, DSi, or 3DS.