First off, I've contributed some code to the Mono guys to make XSP its own library. You can get it as part of the 1.1.8 release. I'm still working on getting a few more additions to the library, but it has all of the basics to get you going.
Download the install from http://www.mono-project.com/Downloads
and get the Mono.WebServer.dll in the Mono-1.1.8\lib\mono\1.0 directory.
With that in mind, here's some code:
XSPWebSource websource=new XSPWebSource(IPAddress.Any,Port);
ApplicationServer WebAppServer=new ApplicationServer(websource);
Console.WriteLine("Mono.WebServer running. Press enter to exit...");
The path should point at your ASP.NET files. That should do it...Enjoy!
NOTE: The API is probably not 100% stable, so you might need to "tweak" your application when a new Mono release comes out.