function doit(){
            var parts = Array("ver-major", "ver-minor", "ver-build", "ver-revision");
            
            //try firefox/non-IE version.
            var nav = navigator.plugins["Silverlight Plug-In"];
            if (nav) {
                for (var i = 0; i < 4; i++) {
                    document.getElementById(parts[i]).innerHTML = parseInt(nav.description.split(".")[i]).toString();   //getControl().isVersionSupported("2.0"); 
                }
            } else {
                //try the IE one now.
                try {
                    var control = new ActiveXObject('AgControl.AgControl');
                    //the following would be faster with a binary search, but this is "fast enough" for now. 
                    var vers = Array(1, 0, 0, 0);
                    loopMatch(control, vers, 0, 1);
                    loopMatch(control, vers, 1, 1);
                    loopMatch(control, vers, 2, 10000);
                    loopMatch(control, vers, 2, 1000);
                    loopMatch(control, vers, 2, 100);
                    loopMatch(control, vers, 2, 10);
                    loopMatch(control, vers, 2, 1);
                    loopMatch(control, vers, 3, 1);
                    for (var i = 0; i < 4; i++) {
                        document.getElementById(parts[i]).innerHTML = vers[i].toString();
                    }
                } catch (e) {
                    document.getElementById("vers-discrete").innerHTML = "Silverlight not installed.";
                }
            }
        }
        function loopMatch(control, vers, idx, inc) {
            while(IsSupported(control, vers)){
                vers[idx]+=inc;
            }
            vers[idx] -= inc;
        }
        function IsSupported(control, ver){
            return control.isVersionSupported(ver[0]+ "."+ ver[1] + "." + ver[2] + "." + ver[3]);
        }
        
        doit(); 

EDIT: that is the code they are using as of this posting