While trying to do some code-behind manipulation of a page I had trouble accessing the controls contained by an UpdateProgress control.  On the same page I have an UpdatePanel and I'm able to use FindControl() to locate controls contained by it in the CreateChildControls event (or any other event for that matter). Not so with the UpdateProgress control!  No matter what I tried the UpdateProgress control would not find any controls.

I finally stumbled upon a tip that you needed to do this in the page's PreRenderComplete event.  Like this:

void Page_PreRenderComplete( object sender, EventArgs e )
{
                Image ImageProgress = (Image)UpdateProgress1.FindControl( "ImageProgress" );
                if( ImageProgress != null  )
                {
                    //Do something with the image
                }
}

ASP.net 2.0, AJAX, UpdateProgress