Apple has released a new Security Update, 2011-03, for Mac OS X 10.6. This update provides new definitions for the File Quarantine function, to cover the recently active MacDefender malware; it also sets up an automatic daily update of the definitions. Further information is in the update description; the update itself can be installed via the built-in Software Update function, or downloaded here. A description of the File Quarantine function is available here.
I recommend installing this update as soon as you can.