There are NO known virus for Mac OS X!
I don't care about the reason behind that lack of virus (obscurity or because of OS security). I just care about the fact that there are no need to run antivirus software on Mac OS X.
The antivirus applications available for Mac OS X only check for Win32 viruses afaik, and only makes sense in a mixed Mac OS X and MS Windows environment.
Yes, there are known viruses for Mac OS X. And as Macs popularity continues to soar, there's going to be some major security breaches in the future.
As stated above, OS X's security is because of its obscurity.