There's an old joke that I find very apt here... Two people are out walking and come across a Cheetah. As the Cheetah eyes them, one slowly bends down and tightens his shoes. The other says to him "dude, you can't outrun a cheetah!". He replies "I'm not trying to outrun the
cheetah."
OSX doesn't have to stay ahead of all the criminals out there trying to penetrate the system. They merely have to stay ahead of their competition, and those criminals will go after the easier, more profitable, pray. OSX will remain relatively free of threats so long as Apple aggressively counterattacks each one quickly and efficiently, taking the profit out of it.
For those moaning that this is the death of OSX... lets do a quick comparison. So far this month (and it's only the second day of the month), OSX has had 1 new variant of malware. Windows, according to McAfee's virus definitions, has had over 30 new viruses discovered today alone. go take a look yourself:
http://home.mcafee.com/VirusInfo/ThreatActivity.aspx