If you can run AVRUSB on the arduino, yes, but it's not the case yet.
I couldn't quite parse that, did you mean you thought it wasn't currently possible to run AVRUSB on the Arduino, or that it's not currently possible to implement the Wii nunchuck mouse on the Arduino?
Either way, the original poster might want to look at the August 19th 2008 entry in the Arduino/AVRUSB project log
where I link to a contributed USB mouse implementation.
I know at least one person on IRC is working on some Wii controller Arduino USB integration.