buenas creo que es esto lo que buscas.. mas info en:
http://www.quirksmode.org/css/opacity.html
.opaque1 { // for all other browsers
opacity: .5;
}
.opaque2 { // for IE5-7
filter: alpha(opacity=50);
}
.opaque3 { // for IE8
-ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Op acity=50)";
}
IE compatibility note
If you want opacity to work in all IE versions, the order should be:
.opaque {
-ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Op acity=50)"; // first!
filter: alpha(opacity=50); // second!
}