Brick Labs

Jens Friedrich has posted detailed steps on how to connect the Wii Nunchuk to your NXT.  He uses a couple of resistors, a diode, a spliced NXT cable and the Lego Accelerometer Sensor from HiTechnic.

Here is a YouTube video of the Nunchuk in action…

And the full forum writeup is here: solved: direct connect Wii Nunchuk over I2C with NXT

If you read to the end, there is an alternate method you can use to connect the Nunchuk to the NXT that doesn’t require any hacking of the Wii Nunchuk itself.