IIS ASP.net, by default, prohibits service of many file extensions, .cs is one.
So the request is being killed by IIS ASP.net before it hits the REST handlers.
This can be fixed fairly easily.
%SYSTEM%\Microsoft.NET\Framework\v2.0.50727\CONFIG\Web.Config
....
<httpHandlers>
<add path="*.cs" verb="*" type="System.Web.HttpForbiddenHandler" validate="True"/>
....
And of course this is just my supposition. I am not privy to the inner workings of the SO servers.
But I do have a bit of experience with how an HttpWorkerRequest behaves.
If anyone needs a better dev server than the one that comes with VS08 or VS10 you can get one here