Inside the remote is an accelerometer that communicates with the Wii through bluetooth. To receive the signal, you will have to purchase a bluetooth shield for Arduino available online or at some Radio Shacks. Does it have to be a Wii remote? You could purchase an accelerometer at Radio Shack and make your own controller circuit...Anyway, here is some info on the bluetooth communication:
"It is built around a Broadcom BCM2042 bluetooth System-on-a-chip, and contains multiple peripherals that provide data to it, as well as an expansion port for external add-ons. The Wii Remote uses (and, at times, abuses) the standard Bluetooth HID protocol to communicate with the host, which is directly based upon the USB HID standard. As such, it will appear as a standard input device to any Bluetooth host. However, the Wii Remote does not make use of the standard data types and HID descriptor, and only describes its report format length, leaving the actual contents undefined, which makes it useless with standard HID drivers (but some Wiimote Drivers exist). The Wii Remote actually uses a fairly complex set of operations, transmitted through HID Output reports, and returns a number of different data packets through its Input reports, which contain the data from its peripherals."
This link might help: http://wiibrew.org/wiki/Wiimote#Bluetooth_Communication
Good luck with your project!!