Seems that everywhere I go these days I hear users saying that they are going to convert to a Mac because Windows is such a pain. Even scarier is that they believe that with a Mac they won't need anti-virus or patching! Ah, hello, who told you that? Ah those Apple ads is their reply.
Firstly, those Apples ads are exactly that, advertisements to get you to buy the product. They are paid for by the people supplying the stuff not some independent third party. Of course they are going to tell you want you want to hear. They want you to buy the product. So even before we start credibility from these ads = 0! (but they are funny - see the latest ones here. Especially check out the one called Podium in light of my previous post on Vista)
Next, all hardware and software is developed by human beings. Yes, they are generally smart than the average human being but they are still humans. They can't foresee every ramification and variation that their product will be exposed to. So no matter what is that is developed by humans, it is subject to flaws and these flaws need to be addressed with updates and patches (Mac included).
Next, the bigger the market share the bigger the target. If you only have 10% of the market why, as a bad guy, would I bother writing something to attack you? I get much greater chances of return if I attack the other 90% of the market. However, as that market share increases then I begin to reevaluate my strategy. This is even truer if you propose that the more uninitiated users are moving towards something like the Mac. As a bad guy if more uninitiated users are there then my potential return is even greater so I am going to devote more time to attacking that segment.
I could go on and on. I also acknowledge that in many ways Macs are better for users BUT don't believe for a second that they are not vulnerable and shouldn't be protected in a way a PC is protected. If you don't believe that then you are playing Russian roulette, because it is only a matter of time before you get hit.
For a good article on the overall issues of Mac security click here.