Unit tests have different requirements than production code. For example, unit tests may not have to be as performant as the production code.
Perhaps it sometimes makes sense to write your unit tests in a language that is better suited to writing unit tests? The specific example I have in mind is writing an application in C# but using IronRuby or IronPython to write the tests.
As I see it, using IronPython and IronRuby have several advantages over C# code as a testing language:

Mocking can be simpler in dynamically typed languages
IronPython has less verbose type annotations that are not needed in unit tests
Experimental invocation of tests without recompilation by typing commands at the interpreter

What are the tradeoffs in using two different languages for tests and production code?
One obvious potential problem is a confusing debugging experience. I haven't personally tried to debug across languages in .NET - what happens if you step into C# code from IronPython?
The other problem is that anyone wanting to develop on your code base has to know both languages.
What are the (dis)advantages of writing unit tests in a different language to the code?