Dave,
I'll take a stab at this (thats all it will be). Generally, heat is transfered to air at the surface. For example, our weather comes from the sun heating the surface, the surface heating the air, the air becoming lighter and rising, etc, etc. The boundary layer is largely stagnant, yes? If you heated the surface enough, the boundary air would heat and rise, potentially upsetting the boundary layer.
My guess, though, is that its 'no factor' in our automobile.