> I can't name a single big or medium sized group using Mono besides Novell itself.

What about Unity (http://unity3d.com/) and all the games based on unity. I'd say that is at least a medium size group.

Edit, links to relevant information Unity's website:

* List of some of the games made with Unity: http://unity3d.com/gallery/game-list/

* Where it says that unity is Mono based: http://unity3d.com/unity/engine/programming