If you are thinking about giving MonoRail a try for ActionPack inspired .net web development, you probably should install the Visual Studio Web Application Project Option and add-ins first.
The MonoRail msi includes a few Visual Studio templates (Castle Active Record Project, and Castle Monorail Project). When attempting to use the template to create a new project of type Castle Monorail Project, you will need to be able to support Web Application Projects (not just the standard default .net 2 Web Projects). If the Visual Studio Web Application Projects add-ins are not installed (like the machine I was trying out MonoRail on), then when creating a new Castle Monorail Project from the template, you will probably get an error saying that you are attempting to open an unsupported project type (MRProject.csproj).