Actually, portions of the SR-71 skin were corrugated, much like the older (and much slower) Ford Tri-Motor, because the high temperatures of Mach 3+ flight caused the skin to expand and contract dramatically throughout the flight profile. This corrugation was actually against the wishes of the aerodynamicists, who would have preferred a smooth surface.

http://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird