Would this be an appropriate time to discuss dividing by zero in modular arithmetic? Dividing by zero definitely plays an interesting role there. I'm not saying any more until either Ricky or Mathisfun gives me the go ahead, because I'm afraid that going into detail might be confusing and hence a bad idea.
The integers modulo n always form a ring, and in such an algebraic structure, my proof that a*0 = 0 holds. Thus, division by zero again does not make sense. Now you can change definitions around to make it work (for example, having 1/a no longer meaning "the multiplicative inverse of a" would do it), but that isn't exactly modular arithmetic anymore.
Ricky, can you be a bit more simplistic? And if it's not "exactly modular arithmetic" than what is it?