Portal is a great video game, the 3DS is a great handheld. Surely it's a match made in heaven? Turns out it sorta is!
This isn't necessarily 'Portal'. It's an unofficial homebrew port called Aperture Science 3D, but it's as close as dammit. It's also extremely, extremely cool.
According to the developer the game is rough around the edges, buggy, and pretty much incomplete but you can find out more here.