No exceptions. (Unless required by law)
I consider myself to be operating under an assumed confidentiality agreement from the moment of first contact. I'm happy to execute a formal agreement if that is your policy. I do not discuss my client's business with anyone unless directed to do so by the client.
For that reason you won't see company names or logos splashed all over my website. My clients are happy to provide a reference---they tell me so. I can put you in touch with some. They're busy people and this isn't the place to publish their names. You'd want the same treatment, right?
Furthermore, I believe it is unethical to (knowingly) provide my service to any business with whom your business competes for the duration of our working relationship.