I created a marker interface:

public interface ISupportAJAXPostsBacks{}

I added it to my Page..

public partial class MyWebForm : PageBase, ISupportAJAXPostsBacks

I have this check in my PageBase class...

if(this is ISupportAJAXPostsBacks)
{
   ... do some stuff ...
}

If I step through via the debugger, "this is ISupportAJAXPostsBacks" evaluates to true for the initial page load, but evaluates to false when an UpdatePanel posts back on that same page. (scratches head)

What is happening under the covers to cause this and what can I do about it?