If you are running Panther (OS X 10.3), read and follow these instructions right now. Apple will undoubtedly release an update for this eventually, but you should protect yourself in the interim. This Wired article has more information.
Update! The brand-new Paranoid Android from the wondrous Unsanity is another option. From their website: “Note: Paranoid Android is designed to shield you from a different exploit approach than described in the recent Wired article and other sources (we do not disclose the details of the exploit method publicly, but Apple has been notified of the issue). However, it will aid you with the above mentioned exploit as well.” Let’s hope so!