University of Florida Says:
Not an urban legend.
The bugs themselves aren't acidic at first, but when they smash into the car, bacteria in their innards react with the air and create a mild acid that does eat away at clear coat and paint if not removed. The good news is that it's completely preventable...by both washing off their mortal remains or by keeping your car waxed.
It mostly affects vehicles with flat fronts. My father's jeep has pits all over the grille from the little bastards.
My Mustang...not so much.