General consensus of the Nerd World is that when Apple has the computer user base that Windows has (if that ever happens, now it is like 8% last I saw), Mac users will not likely experience the virus and trojan issues to the level of Windows due to the somewhat harder to break into system, but it will be so high that there will be severe gnashing of teeth and thoughts of violence. Once the hackers see the level of mayhem they can create rise to a "suitable" level, watch out!
If hackers can break into highly sophisticated corporation, military, and other highly protected government systems, the average Mac user will stand little chance against them, other than having to purchase, install, and maintain anti virus/trojan/phishing software, like most of us Windows users have been doing for years.
Don't get me wrong, I am NOT a big Microsoft fan, nor am I one of those Mac Haters, as I have never had the chance to even turn on a Mac. Never worked in a company that had more than one or two of them, usually in the Marketing department for creating company marketing materials. Being in Engineering, where I have always used CAD systems pretty much forces one into the Windows world. I still have not seen any of the major CAD systems that support Mac. Many years ago a couple did, but they just could not keep spending many man ours porting over when the user base was miniscule at best.
I stuck with Windows XP for many years, just recently went to Win 7 at home, which is pretty darned good. At work we are still running XP (64 bit for the engineering workstations), waiting on Northrop Grumman to start rolling out Win 7 througout their network. That is planned for the next few months I hear.
Macs are great, and if you use them that is also great. It is good that all you need to do is supported by them. However, Win 7 is also pretty darned good, and from all I read is probably the best release of Windows ever. Most of us working in large companies MUST use Windows, no options exist.
Unfortunately for the past couple years I have been using AutoDesk Inventor Pro 3D modeling CAD, just don't get me started on AutoDesk! About the best I can say is that it is " fairly useable". I can have more hang-ups, crashes, and broken files in one month than I had in several years with a competing package.