It doesn't have to be questions. But must invoke an action that only a human would carry.
Perhaps, a message with "welcome to our website. Please help us improve. Tick this box to take a quick survey" OR "tick this box to close this message"... some kind of an action that only a human can carry out. User will not be able to go past this dhtml msg box without taking an action (close it or take a short survey).
It doesn't have to run 24x7, but must run enough time to gather a good chunk of data (last 500 hits or something like that), and especially during obvious zombi "attacks".
get creative people... we can crack it.