Thanks Ian. I've attached a sample project that doesn't work. You will have to remove any references to FastCgiNet.dll and Fos.dll, and then change the main function to host this app with whatever it is you want (Katana, for instance).
My handler only implements IGet (a file called SimpleIndex.cs), it does not implement IOutput or any other behavior, and there is a view with @handler SampleWebsite.SimpleIndex. I have verified that Get() gets called in the handler, but Simple.Web does not set any response after that.