Basically a Mac version of the Rogue Antivirus crap for Windows. Still pretty nasty stuff regardless.But as the article points out, just like in Linux or if you are using a limited user account in Windows 7:
By and large, Mac users have been able to escape the onslaught of malware that their Windows counterparts suffer from. But every once in a while, a piece of nastiness slips into the wild. The latest offender is a Trojan horse by the name of MAC Defender, which purports to be a virus-scanning application. In fact, it does little more than encourage users to give up their credit card information.
However, social engineering can't be discounted...seems to work well enough with Windows users.
As nefarious as MAC Defender might be, the level of concern over infection remains low: Users must be tricked into downloading and installing the program, as well as entering their administrator password.