if you can avoid setting opacity with jquery, you should do this:
$('input[type=submit]',parent).attr('disabled', true).addClass("disabled");
.button-simple.disabled{
opacity:.4;
filter:alpha(opacity=.4);
filter: "alpha(opacity=.4)";
cursor: default;
}
.button-simple{
opacity:1;
filter:alpha(opacity=1);
filter: "alpha(opacity=1)";
cursor: pointer;
}
.button-simple.disabled:hover{
...
}