Hi,
Your code snippet will work (although I agree that it is better to have an IsInitialized property), what I would like to understand is what case are you using this, to see if it is something that all TDD developers would need or if it is something that is perticular to your project.
Wouldn't doing a MockManager.Init before each test solve this?
Are you putting code in you production area?
Are you creating some test setup code (Like a Mock factory)?