the pokertek machine base is solid cast, weighing in at what i'd guess to be about 150lbs. the center is hollow metal tube (I assume cast aluminum) and the top portion is sheet metal "frame" with a fiberglass outer shell. the entire machine weighs in at a bit over 200lbs and 3/4 of the weight is between 0 and 3 inches off the ground...so it's super stable.
even with the thinner "traditional" cocktail design posted earlier, i think if you threw in a couple 50 lb bags of play sand in the bottom of the cabinet...you'd never have to worry about it being tippy at all.