[Skubent]

Примерно так:

_root.followmouse = 0;

absx = btn2._x;
absy = btn2._y;

mc.onMouseMove = function () {
if (_root.followmouse == 1) {
diffx = _xmouse - absx;
speedx = diffx / 550;
btn2._x += speedx * 4;

diffy = _ymouse - absy;
speedy = diffy / 400;
btn2._y += speedy * 4;
if (Math.abs(speedx) + Math.abs(speedy) > 0.5) {
_root.followmouse = 0;
btn2._x = absx;
btn2._y = absy;
}
}
}

btn2.onRollOver = function() {
_root.followmouse = 1;
}