I wouldn't worry about them.
Macs have only 5% of the computer market, so why would anyone want to waste their time making a virus that would little or no damage? Virus makers are in it for the ego trip, and on a Mac no one would notice their efforts. In other words, no applause, no virus.
Furthermore, all Macs before OS X, had proprietary OSs and proprietary chips (Motorolas) which are a totally different breed compared to the Intel chips and Bill Gates Junk.
Now that the OS X is based on Unix, I strongly suspect that modified Unix virii could easily be a problem for that platform. And with the Intel chips coming, there could be further compromising of security. All of that is too early to tell. However, until something becomes newsworthy about virii on Macs, I will continue running without any virus protection on my Macs (and never connect the other POS computers I have to the internet).
The bottom line is that the best protection against virii is to use obscure computers (read Macs), obscure OSs, and obscure browsers. And let the others fight a constant battle against virii.
mmerlinn
"Political correctness is the BADGE of a COWARD!"