Further to Dundurn's post . ..
From the Nintendo website:
"Can I use the Nintendo DS onboard aircraft, in hospitals, or in airports if it's not in wireless mode?
Operation of the Nintendo DS when it is NOT in wireless mode, may be allowed on aircraft, in hospitals, and in airports just like any other non-wireless device would be. You will know when your DS is in wireless mode when the Power Indicator LED (green or red) rapidly blinks to indicate that wireless communication is in progress. The Power Indicator LED will blink rapidly when you are in PictoChat, playing any game in multiplayer mode, or downloading a game to another unit.
Observe and follow all regulations and rules regarding the use of wireless devices in locations such as hospitals, airports, or onboard aircraft. In order to comply with airline regulations, do not use PictoChat, or play the multiplayer modes of DS games while on board an airplane. Operation in these locations may interfere with or cause malfunctions of equipment, with resulting injuries to persons or damage to property"