CSS Code:
#btn{ background: url(transparent.png) no-repeat; filter: alpha(opacity=0); -moz-opacity: 0; -khtml-opacity: 0; opacity: 0; }
For me it worked to just include the filter property with blank value in jQuery's .animate()function
Maybe this will work for you, too.
$("#btn").animate({opacity:1,"margin-left":"-25px", filter:''});
Additionally you might have to remove the filter property from your button's CSS.