Symantec is offering information on what it calls a Mac OS X proof of concept virus dubbed OSX.Macarena. When the virus code is launched, it infects other files in the same directory, regardless of file name or extension.
So far, it doesnit appear that OSX.Macarena does anything other than attach itself to other documents, nor does it appear that it is being widely distributed.
Symantec stated that its virus definitions have already been updated to recognize the proof of concept.
Keep in mind that since OSX.Macarena is a proof of concept, it represents code that shows one way a potential Mac virus could work. In and of itself, it does not signify an actual threat to Mac OS X, but it does underscore the possibility that someday an actual malicious threat to the Mac could surface.