I\'m using the following CSS to achieve a rounded border on my DIV:
border: 1px solid #999;
border-radius: 6px;
-ms-border-radius: 6px;
-moz-border-radius
While I realise that an answer's been accepted, I wanted to suggest -if you need further advice/support/guidance- that you have a look at Curvy Corners; they have a JS library with examples.
As, of course, do MooTools, albeit with a third-party (so far as I could tell) module from phat fusion. And jQuery has its own third-party corner plugin, from methvin.