Although it is not playable yet, a modder who goes by the username 'Smealum' has mananged to get Portal to run on the DS. He has been working on it for the past 6 months.

Here is a quote:

"This is still extremely early footage so please don’t be too harsh. Lots of debug features are present, including the ability to fly, see portals through walls and move cubes from a distance. Also, keep in mind this was shot in an emulator which provides good but still less than perfect rendering. As such, be aware that most (not all, but still) of the graphical glitches you can see in this video don’t happen on hardware (mostly the portal transition is a lot nicer on an actual DS)."

Source (Official site)