For now, yes.
The current Mac operating system is Mac OS X. So far, viruses have only been created that the user has to say 'yes, do all that bad stuff' in order for it to happen, and they also don't spread.
However, that doesn't mean that they won't exist at some point. It's just that Mac OS X is a UNIX OS, which has a much better security model than Windows does (even Vista for the most part), and so you get fewer viruses being able to actually spread and do damage on GNU/Linux, Mac, etc.