I get myTrackingObject is undefined when using Silverlight.isInstalled("2.0") on an IE6 without silverlight installed. This was with the version of silverlight.js released on Oct 20 2008.
On FireFox 3, I get the same "Bad NPObject as Private data" error inside the file SilverlightMedia.js. Is it possible to find the bug in that file as well? I'm using Encoder 2 and having problems...