Personally, I use IoC as some sort of structure map of my application (Yeah, I also prefer StructureMap ;) ). It makes it easy to substitute my ussual interface implementations with Moq implementations during tests. Creating a test setup can be as easy as making a new init-call to my IoC-framework, substituting whichever class is my test-boundary with a mock.
This is probably not what IoC is there for, but it's what I find myself using it for the most..
