I must agree that, whilst a complete Mac newb (only just switched) and have nowhere near enough technical knowledge to give any form of informed opinion on it, the fact that people have the belief that it is impossible to get viruses on a Mac worries me a little (although I am running without AV atm too).
I am always cautious of over (or misguided) confidence in anything, and am concerned that some clever so-and-so almost certainly will (at some point) work out a way to do something nasty to your Mac (unfortunately malicious-hackers and virus writers aren't stupid people...warped, perverted, anti-social, sickos yes...stupid no) .... and when they do, it will catch the community completely unaware and unprepared.
Living in a Windows environment is like being in virus warzone...constantly alert to viruses/spyware/malware, protecting yourself against them preparing for the worst etc.
Living in a Mac envoronment is oh so very tranquil by comparison....I am just concerned that I (and others) will not spot the cunningly disguised landmine as I (we) inadvertently step into the advancing warzone whilst wandering around in Mactopia!
....err...sorry went off on a bit of a tangent there.....i'm having an odd day at work!