Having recently enlisted in the U.S. Army (I'm bound for BCT soon), I have stumbled upon the true origin of "hooah." I asked my Staff Sergeant if I had it right and he confirmed it for me. It's very simple, really.

In the Army, and in the military in general, there is a need to do things, but not right now. Hence the phrase, "Hurry up and wait." Pay special attention to that phrase.

Hurry Up And Wait = H.U.A.W. = "Hooah"

Now we know and knowing is half the battle. Or something like that.

In actuality, though, it really stands for "Heard, Understood, Acknowledged", which is a common phrase used in commo/radio situations.