Macs lack any security measures against viruses. As hackers find more and more exploits on older OSes that are not regularly updated, more viruses and security holes will appear.
FUD-spreaders have been saying that for years. Meanwhile the installed base of iOS--which is a trimmed-down version of OS X--now exceeds 20% of the entire installed base of Windows. And the doomsday scenario you describe just ain't happening. There are architectural reasons for that, and it probably
won't happen.
Right now the most significant malware vulnerability in the wild for the Mac is the possibility of a user's willful installation of software containing malicious payloads. So, don't do that. There is also a Java exploit out there which can target folks with non-up-to-date installations of Snow Leopard. So, take advantage of Apple's frequent updates.
And that's about it. On the iOS side, the walled-garden app environment on top of a very-well-designed architecture has kept users and devices incredibly secure. That's what the future looks like.
I like Lion a whole heck of a lot and recommend it highly. But there are folks who stick with Snow Leopard for legitimate reasons. These include needs for Rosetta and other app-compatibility issues. SL is a truly great operating system and will be supported well by Apple for years hence, including frequent security updates. There's absolutely nothing wrong with it, and those of us who advocate for Lion aren't contending there is.
But it will be left behind as new features are rolled out, and being current on one's OS version will increasingly be required in order to install and use emerging software. That's already occurring.
But viruses and security holes... nope, not happening. Especially viruses, which are formally defined as a self-replicating breed of malware. There are no repeat no viruses in the wild for OS X. And again, there are architectural reasons for that, and it's not likely to happen.