Is PIE being loaded? (e.g. check your server logs). Is it being served with the correct MIME type? Check out the known issues first and confirm that all is OK.

Does it work before you hover the mouse over it (and also, without loading your JavaScript)?

I can confirm it's not working for me either.
I know PIE is loading, because it's rendering the drop-shadow correctly.

edit: nm, I think I found it. "only the shorthand versions of those properties will be recognized."

I've also not been able to get border-radius to work. I'm using more or less exactly what they use in their example, just with the dimension adjusted.

As above, the shadows work.

Here's what I'm using (shadows work, radius doesn't):



... also, doesn't seem to matter where the behavior line is placed, before or after.

EDIT:

I've figured out that, at least on IE8, you can have shadows or you can have rounded corners, but you can't have both.