Is there some sort of library that has all the code possible in it?
A library that has code to read any device or just the Wii controllers?
How boring would the world be if the former was true! 
But, as with the latter, if you Google "arduino " then you'll often find that no matter how strange your idea, someone has got there first.
In the case of the nunchuck -
http://www.windmeadow.com/node/42