IE9 is apparently able to handle rounded corners by using the CSS3 standard definition of border-radius
.
What about support for border radius and>
You can also use CSS3 PIE to resolve this issue:
http://css3pie.com/
Of course, that might be overkill if you're just depending on a single element with rounded corners and a background gradient, but it is an option to consider if you're incorporating a number of common CSS3 features on your pages and want easy support for IE6+