If you haven’t already made the leap to Java 5.0, you’ll want to at least update the version you’re using. The just-released versions 1.4.2_06 and 1.3.1_13 plug a security hole in the Java Plug-In for Windows, Linux, and Solaris, whereby a malicious applet may gain access to your local file system and do evil things. Java 5.0 is not affected.
Hopefully Sun will put the update on Java.com and the automatic Java Update system soon. They seem to be dragging their heels a little, perhaps as a “soft launch” to ensure the update doesn’t produce any serious side effects.