Apple has released a security update to plug a number of holes that allowed malicious software to run on a user's Mac outside of the Java sandbox.
The Tuesday update for OS X Lion and Mac OS X 10.6 is said to fix "multiple vulnerabilities in Java 1.6.0_29" that could allow a piece of code to be run just by visiting an offending webpage.
From Apple's support page document:
Description: Multiple vulnerabilities exist in Java 1.6.0_29, the most serious of which may allow an untrusted Java applet to execute arbitrary code outside the Java sandbox. Visiting a web page containing a maliciously crafted untrusted Java applet may lead to arbitrary code execution with the privileges of the current user. These issues are addressed by updating to Java version 1.6.0_31. Further information is available via the Java website at http://www.oracle.com/technetwork/java/javase/releasenotes-136954.html
The OS X Lion version of the update weighs in at 66.9MB and the Mac OS X 10.6 download comes in at 79.7MB. Both can be downloaded through Apple's support pages or via Software Update.