My wife is a maths teacher, and I have a new DS lite. This means that she now has a DS fat, and we can play Brain Training competitions against one another from our single cartridge.
Inspired by a bit of ad-hoc wireless play (during which she soundly thrashed me several times), she took her new DS into school, and set the kids a competition to see who could get the most Brain Training sums correct in the shortest amount of time – using a webcam to project the game as they play onto the whiteboard. Obviously this generated a great amount of excitement and it was sums-a-gogo for the rest of the lesson.
At the end of the lesson she asked the kids to bring in their own DS the next day, if they had one – eight did and they played the competition against one another. Absolute genius. Talk about next-gen classroom interaction and introducing games as a teaching method!