Actually there's no simple way to round corners in IE via css; you can try using one of the many javascripts but most of them adds a huge amount of DIVs to do that and it's quite heavy for performances...

I found this that may be of interest:

http://code.google.com/p/jqcanvas/

use jpeg rounded images in a table that is most sophisticated way to get the rounded corner rather to use any JavaScript or CSS methods.

currently the best pratice would be to use an .htc script like css3pie, beacuse it's fairly simple to use. You can also try the jquery rounded cornes plugin.