Apple has released Java 10.5 Update 5. There's no corresponding Snow Leopard release; the buggy versions of Java are 1.6.0_13 and 1.5.0_19; the update takes it to 1.6.0_15 and 1.5.0_20, which Snow Leopard may already have.

This can be accessed by using Software Update from the command line or the GUI. Since these include security updates which are remotely exploitable, updating should be of paramount importance.

There's no news yet about any updates for 10.4 – this has the same vulnerability though. The updater doesn't appear to have any restrictions on the version of the OS, and the URL is JavaForMacOSX10.5Update5.pkg, so if you've got a 10.4 era Mac, you might want to give that a download and see if it'll install on your Mac or not.