Virus on the Mac?...

Well kind of... it is malware and it relies on tricking the user to hand over passwords and credit card information.

The real bad news however, apparently Apple has enough market share that the bad guys are now targeting Mac users. And even worse, we Mac users may be better targets for this kind of attack as we have become complacent over the years and are more likely to have a false sense of security.

With that all said, the big news in May was the Mac Defender malware. Basically it did what windows users have been aware of for a long time now... it tried to trick you into thinking you had a virus, and then to give it permission to install itself and then ask your for a credit card number to pay for the "software".

Long story short... the fake antivirus software IS the virus. There is a good explanation here at arstechnica if you want to learn more.

Apple has also posted a knowledge base article here about this as well including instructions on how to remove it if you have fallen victim. 

Basically... don't give your credit card out and change your safari settings to NOT open "safe" files after downloading.