Apple is going cautiously with this, mainly courting small business, scientific and supercomputing. You're not going to see a big switch like that until Apple starts wooing the PHBs away from the Microsoft reps. It's also not going to happen until the large IT contractors stop pushing Windows on their clients.
Given the scenario of a company having a disaster that renders their corp. 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?