Detecting a page close using GetCallbackEventReference IIS6/IIS7 Issue

I have a popup page that when closed,  initiates a server side function that updates a DB table.  After looking at countless posts, I was able to use the following code (partial) to generate the callback when the window closes.
The problem I'm having is that it works great on a server with IIS7, but the event doesn't fire on one with IIS6.
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If Not IsPostBack Then
       Dim callBackReference As String = Page.ClientScript.GetCallbackEventReference(Me, "arg", "LogOutUser", "", True)
        Dim logOutUserCallBackScript As String = "function LogOutUserCallBack(arg, context) { " & callBackReference & "; }"
       Me.ClientScript.RegisterClientScriptBlo ...