Apple have released an update that fixes the Flashback Trojan vulnerability:
Got to Software Updates and install it as soon as possible -
This Java security update removes the most common variants of the Flashback malware.
This update also configures the Java web plug-in to disable the automatic execution of Java applets. Users may re-enable automatic execution of Java applets using the Java Preferences application. If the Java web plug-in detects that no applets have been run for an extended period of time it will again disable Java applets.
This update is recommended for all Mac users with Java installed.
For details about this update see: http://support.apple.com/kb/HT5242