Z-index broken in IE8?

This code works in every other browser I've tried, except IE8.

IE8 appears to ignore the z-index - and the pop-up becomes a pop-under.

It's in the right place, just renders underneath the thumbnail.

Anyone?

Thanks!

HTML:

<a class="thumbnail" href="#thumb">
    <img src="comic_a3_thumb.jpg" height="300" width="212" border="0"
         style="float:right; margin-top:10px;margin-bottom:10px;"
         alt="description" />
    <span>
        <img src="/images/comic_a3_popup.jpg" />
    </span>
</a>

CSS:

.thumbnail{
    position: relative;
    z-index: 0;
}

.thumbnail:hover{
    background-color: transparent;
    z-index: 50;
}

.thumbnail span{ /*CSS for enlarged image*/
    position: absolute;
    background-color: lightyellow;
    padding: 5px;
    left: 0px;
    border: 1px dashed gray;
    visibility: hidden;
    color: black;
    text-decoration: none;
}

.thumbnail span img{ /*CSS for enlarged image*/
    border-width: 0;
    padding: 2px;
}

.thumbnail:hover span{ /*CSS for enlarged image on hover*/
    visibility: visible;
    top: -140px; /*position where enlarged image should offset horizontally */
    left: -500px;
}