Mossberg answers a question in his mailbox column today, including the security threat posed by viruses to Apple's computers (unless you choose to run Windows on it, then you're just as vulnerable). His assessment also applies to computers running Linux. The common misconception is that virus writers don't target these platforms because they are not very popular. While this is true to an extent, it's a disingenous argument because it ignores the fact that Unix-like systems (Max OS X is based on BSD) it's much harder to run malicious code because the computer by default does not use the super-user or Adminstrator account. Its also harder to have a program execute without user intervention or permission, and even if a user does run a malicious program, it'll only have limited access to the system and its files.
As of today, there have been exactly two documented, successful pieces of malicious software -- viruses, trojan horses, worms -- that affected users of the Mac OS X operating system, since it was released in 2001. And these two failed to spread much, affecting probably a few dozen people, and doing no harm. I expect there to be a small number of additional Mac viruses this year.