Apparently by shaking your PS3 controller while playing the upcoming Ninja Gaiden Sigma II you can make your characters boobs bounce around all crazylike (which helps explains THIS advertisement). Also, I'm more than a little disappointed we didn't have this feature for the original NES. I'm looking at you, Princess Toadstool circa Super Mario 2. You too, Mario.
Thanks to Pedro, who makes boobs bounce the old fashioned way: with a Ping Pong paddle.