I learned today that Internet Explorer has an API for detecting whether the user has set your website as his or her homepage.  More details are available here:

http://msdn.microsoft.com/de-de/library/ms531418%28v=vs.85%29.aspx

http://samples.msdn.microsoft.com/workshop/samples/author/behaviors/homepage1.htm

 

I made a quick Fiddle to demonstrate:

http://jsfiddle.net/GY5Ha/

 

Worth noting is that the isHomepage method will always return false if the document that is calling the isHomepage method is not on the same domain.  So my Fiddle will always alert false.