[nbevans]

It's wrong because your unit tests, just like your application, should not be aware of the presence of an IoC container except at the absolute composition root.

Your unit test libraries should have their own composition roots of their own and it is at this level, in configuring the IoC "modules" (as they are often called) that special mock services would be provided.

If your test cases are littered with crap concerning your IoC container, re-binding stuff, then something is badly wrong. Sadly, pretty much all projects I've seen that use IoC end up writing their tests in this way. Someone wasn't thinking.

Your tests should be treated with the same adherence to SOLID principles as your application code. They are first class citizens of your code base. So why dirty your tests by having them coupled to your IoC container?