MyReportServerCredentials

using System;
using System.Data;
using System.Configuration;
using System.Net;
using System.Security.Principal;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using Microsoft.Reporting.WebForms;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Init(object sender, EventArgs e)
    {
        ReportViewer1.ServerReport.ReportServerCredentials =
            new MyReportServerCredentials();
    }
}

[Serializable]
public sealed class MyReportServerCredentials :
    IReportServerCredentials
{
    public WindowsIdentity ImpersonationUser
    {
        get
        {
            // Use the default Windows user.  Credentials will be
            // provided by the NetworkCredentials property.
            return null;
        }
    }

    public ICredentials NetworkCredentials
    {
        get
        {
            // Read the user information from the Web.config file. 
            // By reading the information on demand instead of
            // storing it, the credentials will not be stored in
            // session, reducing the vulnerable surface area to the
            // Web.config file, which can be secured with an ACL.

            // User name
            string userName =
                ConfigurationManager.AppSettings
                    ["MyReportViewerUser"];

            if (string.IsNullOrEmpty(userName))
                throw new Exception(
                    "Missing user name from web.config file");

            // Password
            string password =
                ConfigurationManager.AppSettings
                    ["MyReportViewerPassword"];

            if (string.IsNullOrEmpty(password))
                throw new Exception(
                    "Missing password from web.config file");

            // Domain
            string domain =
                ConfigurationManager.AppSettings
                    ["MyReportViewerDomain"];

            if (string.IsNullOrEmpty(domain))
                throw new Exception(
                    "Missing domain from web.config file");

            return new NetworkCredential(userName, password, domain);
        }
    }

    public bool GetFormsCredentials(out Cookie authCookie,
                out string userName, out string password,
                out string authority)
    {
        authCookie = null;
        userName = null;
        password = null;
        authority = null;

        // Not using form credentials
        return false;
    }
}