Sorry to send you on a wild goose chase, the problem is NOT CRLF issues.
The problem is in your HTTP headers; you're sending the request as:
Content-Type: application/x-www-form-urlencoded
The correct Content-Type for JSON-RPC over HTTP is:
Content-Type SHOULD be 'application/json-rpc' but MAY be 'application/json' or 'application/jsonrequest'
I'm not sure what .net is doing with the body of the request, but by the time it gets to Bitcoin/PostBin, it has x-www-form-urlencoded it enough to confuse the heck out of them.
When you get this working, please update this thread, and for extra positive karma, add a .net section to the Bitcoin JSON-RPC wiki page.