Nintendo has discussed offering demos for the 3DS as part of their update this past November. In typical Nintendo fashion, they're true to their word, just a little later than expected. 3DS demos will start popping up in the 3DS eShop starting tomorrow with Resident Evil: Revelations.
The next demo on the list should be Mario & Sonic at the London 2012 Olympic Games, currently slated for January 26th. Later down the line, we should see Metal Gear Solid: Snake Eater 3D and Rayman Origins. Neither have release dates for the demos.