A Japanese company named Gametech have released
a handheld Nintendo Famicom clone. (For those not in the know, the Famicom was the Japanese game console rereleased in the west as the Nintendo Entertainment System.) It's about the size of a modern handheld game console and takes full-sized Famicom cartridges (which are shaped somewhat differently from the NES cartridges sold in the west, but an adaptor is available). It's not clear how legal it is, though given that it's on the Japanese market, they'd probably have an excuse of some sort (quite probably unlike a different handheld NES clone sold in China, and using miniature copies of Nintendo cartridges). The page says that it's of quite good quality, though given that they're trying to sell them, they would. (via gizmodo)
(This is from the Lik-Sang site, who also sell nifty and bizarre things like discontinued handheld game consoles and pencil cases shaped like vintage game controllers and such.)
A bit of a waste of time trying to make the NES portable when mobile phones do the trick. And with no cartridges.