...a freebsd virus that can run on Mac OS X is just as good as one that targets Mac OS X.
The problem with this is that FreeBSD uses ELF binaries, and Mac OS X uses Mach-O binaries (not to mention that almost all Macs are still PowerPC based systems, and the PowerPC port of FreeBSD still very alpha and not in widespread use). Therefore a Mac user would have to recompile the FreeBSD virus before it would run on his/her system. This would probably require a fair amount of social engineering, not to mention some moderately detailed instructions.