Now that Apple's on Intel they're doing just fine. Enterprises have run just fine on one-vendor hardware for years, like Sun.
HQ unusable, and the need to get, say 2000 workstations and 200 servers delivered within 48 hours, what kind of shape are they in if it's a Mac shop?
Apple seems to be able to deliver hundreds or even thousands of servers for supercomputing clusters, so they have the capacity. Whether the channels are there for 48-hour delivery, I don't know. Apple is currently aiming at small and medium business right now. But I've never seen that scenario.
And if you are talking about huge enterprise, there are still better options than Windows. Linux with a hardware contract from Dell or HP and support from Novell or Red Hat for example. Even if you want your desktops Windows, at least put the email, calendaring and file sharing on something other than Windows.
Do you maintain the users in Windows, or something else? What the solution for managing single sign-on, and getting the the "other than Windows" to recognize Windows security principles for access control?