What is the significance of James Bond's famous phrase "shaken, not stirred"?

Is there really a difference in the taste of a shaken vodka martini, as opposed to a stirred one? And if there is, why?

Mark Langford, Stockport, Cheshire

The shaken-versus-stirred dispute has run for years in The Last Word. One of our books, How to Fossilise Your Hamster, drew on the first three answers below to explain the differences. However, we have since learned that other chemical reactions may be taking place - Ed

I observed some years ago that if, having first stirred a mug of hot-chocolate one taps the bottom of the mug from within with a metal teaspoon the note given off rises quite distinctly with each tap. I have not reproduced the same effect with tea or coffee. However, any normal ceramic mug and brand of drinking chocolate seems to produce the effect. What, if any, are the special properties of hot-chocolate that produce this rising note effect?