This is a Mozilla-specific option, but it makes for some nice edges on interfaces and IE will roll to square edges without any problem:

.mybox {
    -moz-border-radius: 10px;
    border: 1px solid black;
    background: lightblue;
}

See this handy URL for some other tips and tricks:

http://www.eightlines.com/neil/mozskin/csscommands.html