In IE9, which has just been released, you need to provide a meta tag <meta http-equiv="X-UA-Compatible" content="IE=9" />to enable css rendering border-radius.
<meta http-equiv="X-UA-Compatible" content="IE=9" />
border-radius
Why?
This link: http://blogs.msdn.com/b/ie/archive/2010/06/16/ie-s-compatibility-features-for-site-developers.aspx indicates that the meta tag is not the only way to include boundary radius. Instead, you can change your doctype.
Read the entire article to determine if you want to use the doctype or meta tag.