And right, it is no virus, technically. In fact, classical viruses are a thing of the past; it is all trojans, nowadays. Calling it “social engineering,” however, is misleading since the malware gets installed even if you don’t cooperate. (The difference is that if you say “yes,” it gets installed system-wide. If not, it will still run under the user account.)
Not on OSX.6 or OS.7 systems of the past two years... only older OSX.5 systems and older that have not been updated to the newer Snow Leopard and Lion systems.