Transparent box with non-transparent text css...

    div.box
    {
        /* Any properties you'd want the box to have. */
        /* Would probably be position, dimension type stuff. */
        border: 1px solid #000000;
        width: 500px;
        height: 500px;
        position: relative;
    }

    div.box_contents
    {
        background-color:transparent;
        height: 100%;
        position: relative;
        width: 100%;
        z-index: 101;
    }

    div.box_background
    {
        background-color: blue;
        height: 100%;
        opacity: 0.5;
        filter:alpha(opacity=50); /* IE's opacity */
        position: absolute;
        left: 0px;
        top: 0px;
        width: 100%;
        z-index: 100;
    }

Opacity effect accross browsers

    opacity: 0.5;
    filter:alpha(opacity=50); /* IE's opacity */