/*
    HTML:
    <a href="#"><span></span>My hidden text</a>
*/

a {
    display: block;
    position: relative;
    width: Xpx;
    height: Xpx;
}
a span {
    position: absolute;
    top: 0;
    left: 0;
    display: block;
    width: 100%;
    height: 100%;
    background: url(sprite.png) no-repeat 0 0;
}

CSS sprites still accessible when images are disabled.
As seen on http://ryanroberts.co.uk/_dev/experiments/accessible-rollovers/index.html .
Disabled the "overflow: hidden" part to make the sprite accessible when images disabled AND text is zoomed.

Accessible sprites

CSS