We’ve said before, any computer can be exploited. The questions are how easy is it to package the exploit for mass effect, and how much damage you can do once you get in. But, alas, we’re still waiting for the in-the-wild viruses and worms taking down any decent number of Macs. They do tend to remain in the lab. Also still waiting for the Mac botnets.
And, no, don’t bring out the failed numbers argument. Successful viruses have been written to specifically target systems with a far lower installed base than OS X’s 30 million, more like in the thousands.
OSX passed the 30 million mark three years ago... and Apple has been selling approximately 10 million a year since then... Allowing for retirement and destruction of replaced machines, the current estimate is a little under 50,000,000 OSX Macs in operation.