I have to agree with most everything mark has said.
I do, to a small extent, disagree. But it has been an area not really covered. Hacking. All systems, barring an infection of some sort or poor updating, are very secure. Now I am referring to osx, vista, and ubuntu. As for network hacking, all are very secure, neither are really any better. yes that means osx is not magically better than windows.
Once you allow hands on hacking, osx in the last couple of years has been MORE vulnerable to hacking than windows.
In that arena, it went osx, then vista and ubuntu never got hacked in the 3 days.
Now this was the CanSecWest conventions the last couple of years.
But again, this is not to show that osx sucks, just that it is not superiour to windows or linux. Bottom line still is that if you maintain your systems and do not do stupid things, virus and malware infections are very rare. The user in many cases has to initialize the virus install anyways, usually by opening email, running some "questionable" active X item, or some download that is not scanned first from a less then trustworthy source.