@lesmemphis: If you want to send non-HTML data, surely it's better to use a generic handler (.ashx) rather than a full web form (.aspx)? It would literally just be the following:

<%@ WebHandler Language="C#" %>
using System.Web;

public class YourHandler : IHttpHandler 
{
    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "text/xml";
        context.Response.Write("<CallbackResponse>Accepted</CallbackResponse>");
    }

    public bool IsReusable 
    {
        get { return false; }
    }
}