Basically, the Wii sensor bar is not. A sensor bar, that is. It's two sets of five infra-red LEDs powered from the Wii via that tiny cable, and picked up by a camera in the remote. After that, it works pretty much how you'd expect.
This means that if you can create two IR sources and place them in the appropriate place near your screen, you can use the Wii without having the sensor bar connected. This is good if, for example, you have a projector - it is very tricky to both connect the Wii to the projector, and position the sensor bar near the screen.
I don't know why Nintendo didn't include a battery pack with the IR bar, but they didn't.
My experimenting this evening produced one reliable emitter, which was unfortunately burning out the LEDs, and one unreliable emitter which was not. I think with the right resistors, it'd work just fine, but I need to sleep now - I will try to offset the fact that my sleep will be bad by getting lots of it (for me).
Wow, a whole post without sounding paranoid or cranky despite being ill. It's all an act, I assure you.