Does anyone know why a DropDownList, located within an UpdatePanel, will not post back it's selected value when it is set via JScript and a __doPostBack is executed, but it will post back it's selected value if it is manually selected and the post back is handled automatically?

The SelectedIndexChanged event can't only be fired by the client changing the index and isn't fired when the selected index is changed programmatically.  This is always true, not only when it is in an UpdatePanel.  So this is why you are seeing this behavior.

I know the SelectedIndex event can't fire if the changes are done programmatically, that's why I'm manually calling the __doPostBack.  I've figured out how to pull the data directly using either the __EVENTTARGET or __EVENTARGUMENT keys from Request.Form but what I really want is to be able to manipulate the drop down object, not just it's data.