Why would this work in F6, AS1 but not F7, AS2?

[Danger Madcow]

I have the following code which "scoots" a MC to a location with elasticity:

slideY = function(mc, dest) {
	mc.onEnterFrame = function() {
		this._Y = slideMc(this, "_Y", dest, .4, .6);
		if (Math.round(this._Y) == [dest]) {
			trace("slideY done");
			this.onEnterFrame = undefined;
		}
	};
}
slideMc = function(mc, property, destination, inertia, friction) {
	yspeed = ((puppy._y-destination)*inertia)+(yspeed*friction);
	mc[property] -= yspeed;
	return (mc[property]);
}

However this will work fine if I export the swf as Flash 6, ActionScript 1.0, but it will not work as Flash 7, ActionScript 2.0.

Why is this?