Interesting - the DOT regulation doesn't specifically state that steering wheels need to be "round." One might assume that a "steering wheel" must, in fact, be a "wheel," i.e., round. But that is an assumption. (Requiring that "steering wheel lashes" be a specific size, based on their "diameter," also implies a round steering wheel. "Diameter" is defined as "a straight line passing from side to side through the center of a body or figure, especially a circle or sphere.")
But who knows - a strict reading of that rule could disqualify most manufacturers' use of flat-bottom steering wheels because they aren't perfectly "round."