Jonathan Skeet has created a project for developing robots in .NET. Obvious .NET is not supported to run on the App Engine yet. I would guess that Google will not support .NET on App Engine. I don’t know if anybody are working on creating a C# Java compiler, so C# can be run from a Java environment.
To use .NET Jonathan is instead using the Downy to forward all request to the .NET server. Jonathan has created a nice guide to deploy Downy to the AppEngine. I guess that Downy also can be used for other projects, where you don’t want to use the AppEngine for your robots. As I can see Downy just forwards the plain http requests to the .NET server. Downy can also be used to run robots in Ruby or Grails hosted outside the Appengine, which can be necessary for some applications.
The code for the Robot API looks much like the Java API and parses all the JSON using one of standard library for parsing. It looks fairly easy to use and you should be able to follow most of the Java tutorials. Expanding the Robot library to more languages, will make it much easier for developer to choose their favorite language.