I think I was wrong in emphasizing training alone as the
only answer to aggression problems. Many cases of aggression
are due to medical reasons. There are 50 know medical problems
that can have aggression as a symptom & 13 different kinds of
dog aggression. Here's a link to some types of aggression;

http://www.allbreed.net/chows/Train/aggrtype.htm

and one article on just one of the medical problems that can
be involved.

http://www.petplace.com/articles/artShow.asp?artID=1807

I think anytime there is a marked change in behavior, a good
checkup be your Vet is a good idea.