The Wiimote is supported in Linux kernels since version 3.1. However, due to the complex design of the controller, the driver exposes each subdevice (IR, accelerometer, buttons) and each extension as a separate device, none of which is recognized as a controller by Steam.

The xwiimote project (available here) can be used to interface with the Wiimote on a higher level. An example on how to use it can be seen in the xf86-input-xwiimote X input driver.

At the very least, support for the controller in “gamepad mode” (rotated left, without using the IR camera) would be ideal.