[CSS] rounded corners

What’s the most efficient way to code rounded corners in CSS? I know the easiest way is to just create 3 DIV’s. But this can make the HTML very sloppy. Is is there some cool convenient way to do it where you you can apply multiple backgrounds to a single DIV? Like:

background1: url(‘middle.gif’) z-index=1;
background2: url(‘leftRounded.gif’) align=left no-repeat z-index=2;
background3: url(‘rightRounded.gif’) align=right no-repeat z-index=3;