[Kikasso]

stop();
var fadeSpeed = 5;
var limAlpha = 55;
function fadeIn(){
 this._alpha += fadeSpeed;
 if(this._alpha >= 100 ) delete this.onEnterFrame;
}
function fadeOut(){
 this._alpha -= fadeSpeed;
 if(this._alpha <= limAlpha ){
  this._alpha = limAlpha; 
  delete this.onEnterFrame;
 }
}
function doRollOver(){
 this.onEnterFrame = fadeIn;
}
function doRollOut(){
 this.onEnterFrame = fadeOut;
}
/// код для кнопки ///
mc._alpha = limAlpha;
mc.onRollOver = doRollOver;
mc.onRollOut = mc.onDragOut = doRollOut;

upd: конечно код не "на" кнопку, а "для" кнопки.