Even if Macs become mainstream, they won't have even near the viruses as Windows. Why is that? A number of reasons:
1: Microsoft Windows programmers are lazy. Microsoft had to hit them on the head with UAC so they wouldn't have their code always run as the equivalent of root every chance they get. Mac developers understand the user/root model. Even though Windows has DEP and ASLR, Windows devs are too lazy to implement it, or even actively break DEP due to lazy programming. Because of this lowest-bidder coding, it isn't a big wonder why there are so many security holes in Windows applications.
2: Mac developers don't crap where they sleep. People have a respect for the OS X platform. This is the same with Linux, BSD, and many others. It isn't considered cool to trash these platforms. Windows doesn't have that, as most developers are only loyal as long as the checks keep rolling in.
3: Users of non MS platforms tend to be more educated. Choosing to buy a Mac is a deliberate choice. It isn't just like grabbing some computer out of a big box store, setting it up and turning it on, and using Windows because it was on the machine. Mac users deliberately purchased their hardware.
4: Macs are built from the ground up for security. While Windows has to have some sort of CPU and resource hogging antivirus utility in memory 24/7, no other platform needs this for basic protection.