Sometimes Mac's are easier to invade than Windows There was a hack fair where the Mac was invaded in two minutes; Windows Vista in 2 hours; and Linux stood up for the entire weekend. All systems were fully up-to-date. One of the problems with Mac's is that, for a lot of times, it's users don't have a choice but to search online for programs, because what's in Apple's repositories won't do the job. I don't where I read it but I think Apple is changing that, and so is Windows. They both will have official repositories in the same way that Linux had for over 20 years.
Yeah signed programs from official repositories is probably one big reason that in the wild Linux doesn't get hacked so often.
Exactly. Mac and Linux both have similar permissions models. Still, IIRC Opera had a serious bug that allowed complete system control if exploited. I wouldn't recommend users to use default Apple browsers, but to use Firefox whenever possible. No FlashPlayer either, and a few security addons just so no script runs by default.