The accelerometers found inside the controller halves are produced by Analog Devices in America and STMicroelectronics in Europe. The two companies are expanding because of the high demand for its products, either by creating new production lines or dedicating entire buildings to Wii component production. Expansion like this would only be possible if it was financially feasible to make such changes, and the success of the Wii means everyone who has a hand in its production is profiting.
Because Nintendo is buying up accelerometer sensors left and right, the increased production of them means that their price is dropping. This allows other electronic devices, such as mobile phones, to more cheaply incorporate them.