Apple rolled out the new Mac Security Update 2011-003 to address threats posed by the latest family of malware under the MacDefender moniker. Keep your system up-to-date with Apple’s built-in Software Update program (Apple menu > Software Update), and if your system is infected by MacDefender, the security update will helpfully nuke it.
After you install the update, consider protecting yourself with some advice that I offered in a previous post.
Stay safe in cyberspace!