I remember having a big discussion with a friend at work and we were trying to work out how lightguns worked before we looked on the internet and found out. It's so deceptively simple - basically just a light sensor and a clever bit of programming. Very neat.
The Wiimote actually works kind of the other way round, the camera is in the wiimote and the 'sensor bar' (bit of a misnomer) is essentially just some IR bulbs - the pointer on screen moves relatively to where the wii mote is 'seeing' the IR bulbs. You can also use candles or regular light fittings spaced out to control the pointer.
Didn't the Steel Battalion controller require assembly too?