This evening, Apple has announced that they will be developing a tool to remove the Flashback trojan horse that has infected over half a million Macs. While Apple responded by releasing an updated version of Java on April 3rd (for both OS X 10.6 and 10.7), users who haven't ran Software Update on their Macs are still susceptible to becoming infected if they visit a website with a malicious Java applet. According to Macworld, Flashback harvests usernames and passwords from web browsing activities.
In response to the widespread infection, Apple is working with ISPs to shut down the servers Flashback uses to communicate with its authors and perform its commands. In addition, Apple is developing and will be releasing a special removal tool focused on removing the Flashback trojan from infected systems. A release date is not known at this time, but in the meantime, you can follow this guide from F-Secure or use this handy tool to see whether you are infected. Instructions are given to remove the malicious program manually. Apple's solution will likely be automatically included in a future Software Update that takes care of the infection for the user.