Apple today released Java updates for Leopard and Snow Leopard. Apple provided very little information about what is included in the Java update, only saying that it delivers "improved reliability, security, and compatibility."
A little research reveals the security aspect addresses a problem that could allow OS X to treat an expired security certificate for a web site as still valid. The second issue addresses the possibility of an untrusted Java applet being able to raise its privilege levels.
The Java update is available from Apple's Software Update service (Apple menu, Software Updates). You can also download it directly from Apple's web site.