There are many cases where you may want to add multiple buttons to hide or show the MS AJAX modal popup extender dialog.

You also may want to intercept close and show events with javascript to do other functionality prior to modal load and following modal close, such as clearing the content of RIA content which will otherwise continue to run in the background (in IE at least) on close.

//don't forget to set BehaviorID ="modalname"
//call showmodal('behaviorid') or hidemodal('behaviorid')

    function showmodal(modalname) 
    {
//do something to get/update modal content here or from server side
      $find(modalname).show(); 
    }

    function hidemodal(modalname) 
    {
document.GetElementById(modalname).innerHtml=''; //clears content
$find(modalname).hide(); 
    }

References
http://blogs.msdn.com/b/phaniraj/archive/2007/02/20/show-and-hide-modalpopupextender-from-javascript.aspx