Contrary to what the preceeding w/u may lead you to believe, you will rarely (if ever) hear this particular Southernism exclaimed in Texas. If you are in Texas, and you hear this, chances are that the person yelling is probably from Arkansas or even worse, a drunken Yankee on vacation. I am not really sure why Texans don't like "yee-haw," but I am sure there are some Longhorned noders out there who can explain...