Just to get this one out of the way, you cannot mock (or stub) a static method, period. Crying or wailing about it won’t help. There is no easy way to substitute in a stubbed implementation for a static method. For that reason alone, I advise developers to never do anything in a static method that calls out of the current AppDomain (HttpContext objects, database calls, MSMQ manipulation, etc.). Even if you need to use a static (or thread local storage or HttpContext storage) field to maintain state, all consumers should access this cached state through some type of abstraction interface for easier testing.
Besides the testability issue, do you really want your application strongly bound to a particular implementation like MSMQ? If you limit the coupling to MSMQ, you can more easily substitute in MQSeries or a call into your company’s new utopian Enterprise Service Bus later. My point merely being that designing to maximize testability often happily coincides with just plain good design or architecture.
Singletons always, always, always interfere with writing decoupled and isolated unit tests. There is a lot of interest in Domain Driven Design right now and one of the key concepts in DDD is that often the domain objects are retrieved or persisted through a repository that manages caching and persistence for a domain class or small family of classes. The repository is stateful across requests or classes, so it is a natural candidate for a singleton. Don’t be seduced by the dark side. If you make the repository a singleton, all of the consumers of the repository (and there will be a bunch) cannot be tested in isolation. You can do something to allow you to mock or stub out the underlying data source classes, but take it from me, this sucks (hard). Your unit tests are dependent upon the implementation of the repository class. This makes your unit tests harder to write because there is more code, and harder to understand because you are investing more of the unit test in setting up the internal state of the repository than exercising the consuming class. For a multitude of reasons, it is always important to make your unit tests intention revealing. Mocking the guts of the repository may end up obfuscating the real intention of a unit test.
Here’s an article I wrote on a TDD-friendly alternative on the StructureMap site for exactly this situation with more discussion on the issue: http://structuremap.sourceforge.net/SingletonInjection.htm.
You should also check out Robert Martin’s article Singleton vs. Just Create One. While you’re on the ObjectMentor site, read everything those guys write, it’s all good stuff.
As an aside, one of my friends does a lot of technical interviews for .Net developers for a very large employer in Austin. His weedout question is “what is the difference between a static and an instance method?” He is continually scratching his head on the percentage of people who wash out on this question. I have no idea what to say about that.