Just spotted this article from ComputerWorld about fake security software on Mac. You'll want to be knowledgeable about this threat. Apple discussions has more than 20 entries on this, with the most recent dated today, 5-2-11.
Jim Hamm sends us this from The Unofficial Apple Weblog with specifics for finding and deleting -- as well as preventing -- the MacDefender app. The article tells us to uncheck "Open 'safe' files after downloading" in Safari Preferences. And the writer closes with, "For those of you who haven't been hit by the MacDefender app, take care while downloading images for the next few weeks."