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?
When building an API or library, I often deliver unit tests as a form of documentation on how best to use the API or library.  Most of the time I build a C# library, I'm delivering to a client who will be writing C# code to use the library.
For documentation sake, at least some of my tests will always be written in the target language.
What are the (dis)advantages of writing unit tests in a different language to the code?