IIS gives 503 after updating .NET 4.0 to beta 2

From my experience with ASP.NET MVC, I've seen that a Default.aspx page is required for IIS to function correctly. I'm using the page that was included in the ASP.NET MVC 1 template. Unfortunately, the ASP.NET MVC 2 does not include this page (to the best of my knowledge), so you should add the following to your project:

Default.aspx:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="YourNamespace._Default" %>

<%-- Please do not delete this file. It is used to ensure that ASP.NET MVC is activated by IIS when a user makes a "/" request to the server. --%>

Default.aspx.cs:

using System.Web;
using System.Web.Mvc;
using System.Web.UI;

namespace YourNamespace
{
    public partial class _Default : Page
    {
        public void Page_Load(object sender, System.EventArgs e)
        {
            // Change the current path so that the Routing handler can correctly interpret
            // the request, then restore the original path so that the OutputCache module
            // can correctly process the response (if caching is enabled).

            string originalPath = Request.Path;
            HttpContext.Current.RewritePath(Request.ApplicationPath, false);
            IHttpHandler httpHandler = new MvcHttpHandler();
            httpHandler.ProcessRequest(HttpContext.Current);
            HttpContext.Current.RewritePath(originalPath, false);
        }
    }
}

Officially I don't think this configuration is supported yet. See the ASP.Net 4 breaking changes whitepaper.

Unofficially, you should be able to fix the problem by changing the trust level in your web.config file from WSS_Minimal to Full. This is NOT recommended though.