This probably shouldn't come as a huge surprise to anyone who has been on a plane since September 11th, 2001, but in a recent article by Yomiuri, we've learned that Japan's Ministry of Land, Infrastructure and Transport has official restricted the use of the wireless multiplayer functions of portable game systems, namely the Nintendo DS and Sony PSP.
The justification behind the rule is the same as virtually all other rules associated with the use of wireless technologies inside airplanes: they may cause interference with cabin equipment. I was under the impression that this kind of thing never actually happened and was just a precaution, but according to the Ministry there have been more than 200 reported incidents since 1993 related to such interference.