Internet Explorer issues with wpad.dat

I am setting up a Squid caching proxy server at work to cache some map data for one of our web apps. My wpad.dat works great in Firefox & Chrome but it does not work correctly with Internet Explorer. Does anyone know how to get IE to properly use a wpad.dat file? My current file is below, I have had it working when I run all data for the site through the proxy, but that is not what I want to do. Could it partially be due to the fact that the web app is a Silverlight app?

function FindProxyForURL(url, host) 
{
    if (shExpMatch(url, "*/ArcGIS/rest/services/*/tile/*") && isInNet(myIpAddress(), "192.168.0.0", "255.255.0.0"))
    {
        return "PROXY 192.168.11.15:3128; DIRECT";
    }
    else
    {
        return "DIRECT";
    }
}