In case you missed it, there's a virus for the iPod. Yep, that's right, your MP3 player is a veritable hotbed of virus activity -- but only if you're running the iPod Linux distribution, and only if you take great pains to make the virus function, since it doesn't really work. We can argue about whether or not this code actually constitutes a virus, but that's not the point I'm trying to make.
The point here is that if it has a CPU, hackers will try to break it, and virus writers will try to write a virus for it. Given that there are probably only a few hundred -- maybe a thousand -- iPods running Linux out there, the fact that someone took the time to write this virus, or malicious code is an example of why Apple detractors clamoring that Macs aren't a target due to the lower market share are all wet. I ranted on MOAB two weeks ago, pointing out that most of their bugs were either local exploits or issues within third-party applications, and there has never been a virus in the wild for OS X, much like there's never been one for Linux. The difference isn't market share, it's the foundation of the operating systems. Given that most virus authors and hackers are in it for the ego, don't you think that there would be a huge incentive to be the first one to write a widespread OS X, Linux, or FreeBSD virus?
If an OS is built on shaky ground, everything layered on top will suffer. This is the position that Microsoft is in now. Apple was in this very position at the end of the last century. They decided to start over, providing a clear upgrade path and supporting legacy applications on the new platform. OS X was developed from BSD and NeXT, built on a foundation that dates back twenty years or more, with the OS base code freely available for download, yet there have been no significant security vulnerabilities in OS X. This isn't due to market share, this isn't due to lack of attention, this is due to proper coding and development. That isn't to say that there are no chinks in Apple's OS armor -- there definitely are -- but the foundation is solid, therefore those chinks aren't likely to destroy the whole shebang. The same is true of Linux, and most UNIX-derived operating systems.