It definately was not a UFO.  It was the United States Air Force's B-2 Bomber (commonly referred to as the Steath Bomber).  Supposedly it was 1 out of the 21 total B-2 bombers the Air Force purchased.

That plane is just plain "sick"!  So friggin cool!  Engines were enormously loud and it's silouette unmistakable.  It happened to be the fly-by for the University of Arkansas vs Kentucky football game.  It is such a terrifying piece of machinery that my 2 year old son got quite scared.

It is amazingly impressive that there are no verticle pieces of the plane.  From my understanding, because it defies normal flight design, the only way that they can keep it from crashing into the ground is through gyroscopes.  It's modern living proof of an engineering masterpiece.