flash actionscript:

This code records the position of the mouse and stores the past position. To
calculate speed, take current_x - past_x or current_y - past_y.

this.onEnterFrame = function() {
//set past position
past_x = current_x;
past_y = current_y;

//set current position
current_x = this._xmouse;
current_y = this._ymouse
}