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?
I think that this is an excellent question and an idea worthy of consideration - particularly in an environment like Visual Studio/.NET where this is easily supported
The plus side - as you suggest - is that you can choose to use a language/tool to create tests that is more suited to creating tests than perhaps the code you are using to create code and for this reason alone its worth a thought.
The down side is, as suggested, that your developers - those creating the tests (and we must remember not to confuse Unit Testing with Test Driven Design) probably ought to be fluent in more than one language (I'd suggest that the ability to be so is fairly important to a good developer but I'm biased!) - and more importantly that you may have to worry about structural differences between the two (though again, if you're talking about .NET languages that should be covered for you).
It gets even more interesting if you go beyond "unit" tests to tests at all levels where the specific capabilities of particular languages may give advantages in building up a test case.
The ultimate question is whether the advantages outweigh the disadvantages... and that's probably somewhat case specific.
What are the (dis)advantages of writing unit tests in a different language to the code?