Japan's Ministry of Land, Infrastructure, and Transport has put its tinfoil hats on and have concluded that that electromagnetic waves emitted by portable WiFi gaming devices posses the ability to interfere with aircraft navigation systems and other devices.
Effectively, this means the Nintendo DS, PSP, and other handheld devices are now banned from being used during flight. Portables without a WiFi function, such as the GBA, can still used however.
Video games banned on Japanese planes