Network World: How Secure is a hosted environment

In a recent NetworkWorld article, Michael Osterman asks “How secure is a hosted environment”. Specifically, he’s talking about external hosting of mail stores in cases where the entire mail operation is outsource or where mail is archived externally.
The article reports on his trip to ZANTAZ and how impressed he was with their physical security. The article would have been better if it had covered other areas of information security. How are these servers protected against attack. Is the operation audited? How do you know those security doors aren’t propped open every other day of the week?