You can do it the old-fashioned way and create images for each of the corners, and then use a
different set of CSS rules for IE.
Here's the site I used to use back when this was the only way:
http://www.roundedcornr.com/
You could also do
crazy things with VML, but that's messy and excessive.
Alternatively, just make sure the site looks good without rounded corners. This is honestly the best option. If the site really needs the corners to look good, chances are they're excessive
