I've heard that cockpit air is completely refreshed every 3-5 minutes, and new air comes from the (first?) compression stage from the engines. That's the reason for the air conditioner system always working, the compressed air becomes too hot.
There's an active thread today covering the emergency landing of a Continental B757 in N.C. "due to smoke in the cockpit due to engine problem".
So if we have smoke in on engine then we'll have smoke in the breathable air?