Webservice C# JSON to Arduino
Yes.

If you're using an Ethernet Shield, you should be able to run a webserver on the Arduino and simple do an HTTP request from your C# webservice to the Arduino.

Check this tutorial on how to write a webserver for arduino using the ethernet shield: http://arduino.cc/en/Tutorial/WebServer

Doing an HTTP request of any kind from C# should be easy enough and is not in the scope of this site.