*puts World of Warcraft Goblin Engineer psuedo-science hat on*
Well it works like this.
The Shield, with its vibranium-iron alloy mix, is not uniformly amalgamated throughout its structure. Because of the relative density differential and shape of the shield, the iron alloy is greater in the mix at the Shield's edge, which minimizes the absorbing properties of the vibranium, thereby allowing it to retain its spring-back ability.
Likewise, the grain of the vibranium in the alloy is more or less at a right angle to the surface of the curvature of the shield, which allows it to absorb shock in proportion to the vector of the incoming force; hence, when a bullet hits it, it ricochets off the side, but significantly spent (which is why there aren't many ricochet fatalities). Also, this is why the Shield can absorb a direct-opposition strike (like the mallet head of Thor's hammer, or the Hulk's fist, but still be a powerful offensive weapon as well, as when Cap uses it as a weapon, he rarely uses it at an angle of straight opposition.
(It should be understood that I'm blathering psuedo-physics like nobody's business, and don't know what the heck I'm talking about - now, where's my No Prize? *snickers*)
I say Dave's explanation should go unapologetically into the wikipedia entry about Cap's shield! All in favor, say "aye!"