Not that I'm overly concerned, but you have to think, if a succesful virus did target the Mac, it would have a pretty easy time of it. Apple users are allways talking about how immune they are to malware threats and thus almost nobody has virus protection.
Of course this assumes that someone come up with an infection vector that can target Macs specifically instead of just blasting out onto the internet in hopes of infecting someone. That works with windows because you are just bound to find a windows machine with that approach. Macs, making up much less of the market, are just that much harder to infect.
Frankly, I think for the forseeable future there just isn't going to be an effective infection vector that would work with the average Mac computer. Of course specifically targeting servers and the like is a different story.