Hi,
Most of the CSS stuff to round corner will not work in Internet Explorer browser. Even JQuery UI have some button controls that is available with round cornor with different theme. Check
http://jqueryui.com/themeroller[
^]
I think
jQueryUI is the best that you can use. It will not work in IE6. Not sure about other IE versions. If you have strong requirement for roundcorner then you can even use images to display it as button.
Thanks.