This one has be baffled. When I go to the page with the border-radius, the curved corner will appear while the page is loading, but by the time the page finishes loading, it disappears. I've tried it on a couple different elements.

Each element has position: relative and z-index:0 defined. And the fact that it's appearing briefly tells me that the PIE.htc file is being called properly.

Any thoughts as to what could cause this?

So in case anyone has the same problem, I discovered the issue. It's a conflict with a googlecode script that's supposed to help IE behave problem. But it's designed for IE6 and below. It's this script:

<script src="http://ie7-js.googlecode.com/svn/version/2.1(beta4)/IE9.js"></script>

I just changed it so the script only loads for IE6 or below (I had it set for 8 and below) and it's fine now. Oddly, it doesn't conflict on IE6 (though that version of IE has it's own problems), so no problems there.