My present employer (who I've finally decided to try and make my former) has a policy forbidding any employee from serving as a reference to any former employee. They are literally not allowed to comment in any way, but are told to forward all inquiries to a phone number which will verify employment (a number that no one actually seems to know).
Ostensibly, this is to protect the corporation from potential lawsuits; but it has a chilling effect on employees who are looking to leave.
So I'd warn anyone to be careful about working at a place like this, as it can make leaving much more difficult. A policy like this isn't something you'd generally think to ask about while interviewing, so I don't know how you'd find out about it ahead of time, but it's probably worth asking about.
I've had companies ask me if I could violate the policy and provide them references anyway, which I'm not a big fan of. As anti-employee as I consider the policy, I think it's important to follow the stuff you've agreed to (and I'm not sure what it says about a company who is that willing to have you violate the policies at your existing company).