The best solution I've found for rounded corners and other CSS3 behavior for IE<9 can be found here: http://css3pie.com/
Download the plug-in, copy to a directory in your solution structure. Then in your stylesheet make sure to have the behavior tag so that it pulls in the plug-in.
Simple example from my project which gives me rounded corners, color gradient, and box shadow for my table:
.table-canvas
{
-webkit-border-radius: 8px;
-moz-border-radius: 8px;
overflow:hidden;
border-radius: 10px;
-pie-background: linear-gradient(#ece9d8, #E5ECD8);
box-shadow: #666 0px 2px 3px;
behavior: url(Include/PIE.htc);
overflow: hidden;
}
Don't worry if your Visual Studio CSS intellisense gives you the green underline for unknown properites, it still works when you run it. Some of the elements are not very clearly documented, but the examples are pretty good, especially on the front page.