By default, the 3DS upscales original DS games to fit the bigger resolution of the new screens. However, if you hold down Select and Start when you select the game from the Home Menu, you can play in the original resolution (256×192). Obviously this might cause problems for touch-centric games, but it’s awesome that Nintendo put this feature in there.
UPDATE: A couple of other DS-mode button combos: Press the Home button and then hold Start and press a direction on the D-pad – Up/Down adjusts brightness, while Left/Right switches the power-save mode on/off (automatic brightness depending on what’s on screen). Unlike the scaling trick, this is listed in the manual.