Hey Zac,

Looks like I kinda lost the source code for this, but not to worry, a little decompile helped, this is all there is to it: 

using System;
using System.IO;
using System.Web;

namespace Cultiv.DynamicRobots
{
    public class RobotsTxt : IHttpHandler
    {
        public bool IsReusable
        {
            get { return true; }
        }

        public void ProcessRequest(HttpContext context)
        {
            context.Response.ContentType = "text/plain";
            var path = HttpContext.Current.Server.MapPath(VirtualPathUtility.ToAbsolute("~/robots.txt"));
            if (File.Exists(path))
            {
                var streamReader = File.OpenText(path);
                var text = streamReader.ReadToEnd();
                context.Response.Write(text.Replace("{HTTP_HOST}", HttpContext.Current.Request.ServerVariables["HTTP_HOST"]));
                streamReader.Close();
                streamReader.Dispose();
            }
            else
            {
                context.Response.Write("");
            }
        }
    }
}

Thanks!

if a site is multilingual, like "mysite.com/de", what should be instead of "HttpContext.Current.Request.ServerVariables["HTTP_HOST"]"?

is working on a reply...