If you define the end state in the class then it should do what you want in the example:
.drop_box {
-webkit-transform: translateY(100px);
-webkit-animation-name: drop;
-webkit-animation-duration: 2s;
-webkit-animation-iteration-count: 1;
}
But if your animation is event driven anyway you will probably end up having to use a bit of JavaScript. The easiest way is to make the adding of the class with the end state in it be what triggers the animation to start.
--edit
See dino's answer for information on the animation-fill-mode
property added in the April 2012 WD.