Passing Arguments to ASP.net UpdatePanel using __doPostBack()

Hello,

I have an asp.net page that contains an update panel and I am trying to figure out how to pass arguments on it when I call __doPostBack().  I have the following function in my javascript that causes the UpdatePanel to perform an Async Postback and refresh itself:

        function UpdateFoodLogPanel() {
            __doPostBack('<%=FoodLogUpdatePanel.ClientID %>', GetSelectedDate());    
        }

This function is called everytime a user selects a new date from a Datepicker control on the page.  Obviously the second parameter of the __doPostBack() is the date which the user has selected.  I need to access this date in the code behind in order to populate the update panel with specific data based on what date has been passed.

In my codebehind I have tried using Request.Params("__EVENTARGUMENT") retrieve the date from the __doPostBack() yet for some reason it is always showing up as a blank string.  The funny thing is that I am using Firefox with Firebug and I am able to access the POST of the response and it is showing that __EVENTARGUMENT does have the proper date inside of it when it is passed.  Another curious thing going on is that the Page_Load event in the codebehind never seems to register IsPostBack as being true, even though the page IS being posted back when the UpdatePanel is refreshed.

If anyone could help me figure out how to get the date in the code behind it would be VERY much appreciated.

Thanks,

Mike