Even if your system was wide open for attacks, but only 1% of the net users used it, and if someone did want to write a worm for it, chances are that it'd still be hard to get it distributed. You've gotta, like, find one of them machines to be able to infect it.

Also, whoever writes them virii must really want to go for that specific group to pick a 1% segment.

This is simple logic and says nothing about Mac security, market share or virus availability, which I don't care to go into detail on since that's a fight without logic and facts.