Is it possible to for a user to add another Mono language to Unity?

I don't think it will be possible to add "native language support" for another language to Unity without a terrible hack.

However, what you can do quite easily is create an assembly (DLL) in any .NET / Mono language which you can use from your Unity scripts. All you need to do that is compile your library separately and copy it into your assets folder. The main limitation is that you can't trivially create MonoBehaviors that way.

However, you could probably create a structure that "links" very simple C# or UnityScript MonoBehaviours into more complex classes written in your .NET language of choice. In other words: You'd keep the actual implementation in your DLL and just call the relevant methods of classes that reside in your custom DLL written in a custom language.

With Unity iPhone, you need to be careful because there's a limitation of how many assemblies (DLLs) you can include in a build. And of course, Unity iPhone currently only supports .NET 1.1 (or at least, it doesn't support generics).

Not at the moment, no ;)