For anyone else who encounters this, I found the issue.  It actually was the AjaxControlToolkit.  Apparently, the newest release does not dispose of controls properly when you have a modalpopupextender inside of an update panel.  As a result, multiple controls of the same ID get created on the page.  This caused me a whole lot of pain.  This error turned out to be only the tip of the ice burg.  Once I rolled back to an earlier version of the toolkit, all my problems went away.

 

Thanks.