You are quite right in that I2C only requires 2 pins of the processor for the signaling and it can be made to work only over 2 wires if a bridge has been used and the device doesn't have the same power source as the processor. However, in practice the devices and the processor share a common Vdd wire for power and a ground wire - total 4 wires. If you cut the cord between the Wii remote and the Nunchuk there are 4 wires - that's all I was attempting to convey.
The specification from Phillips details all the various electrical requirements for those interested, wikipedia does not.
http://www.nxp.com/acrobat_download/...8/39340011.pdf