I looked at this early on in the creation of Stack Overflow and couldn't find any method of creating rounded corners that didn't leave me feeling like I just walked through a sewer.
CSS3 does finally define the
border-radius:
Which is exactly how you'd want it to work. Although this works OK in the latest versions of Safari and Firefox, but not at all in IE7 (and I don't think in IE8) or Opera.
In the meantime, it's hacks all the way down. I'm interested in hearing what other people think is the cleanest way to do this across IE7, FF2/3, Safari3, and Opera 9.5 at the moment..