Well first of all, the reason why this works in Firefox etc, is because those browsers supports the background gradient.
Second of all, there’re a several reasons why this might not work. First is: wrong including. What is the location, the exact location, of the pie.htc file? If the style.css (or something like that) is located in the root folder and the pie.htc is in the pie folder. The including should look like this:
When the style.css is located in the folder stylesheet, it should look like this:
If you applied the above code, and the PIE script still doesn’t work, the server might be the problem. Instead of including PIE.htc, you could include PIE.php. Make sure that the PIE.htc is in the same folder as the PIE.php folder.
If you’re dutch, you might like my blogpost about PIE: CSS3PIE handleiding