IE9 is apparently able to handle rounded corners by using the CSS3 standard definition of border-radius.
What about support for border radius and>
I also got stuck in the same problem n found that IE can't render the border radius and gradient both at a time, it both conflicts, the only way to solve this headache is to remove the filter and use the gradient via svg code, just for IE ..
you can get the svg code by using their gradient color code, from Microsoft this site (specially made for gradient to svg):
http://ie.microsoft.com/testdrive/graphics/svggradientbackgroundmaker/default.html
enjoy :)