Apple recently released Java for Mac OS X 10.5 Update 1 (which adds Java SE 6 to your Mac) after receiving much criticism for omitting it when Leopard shipped:
Discussion threads began to appear in Apple's support forum following the release of OS X Leopard on 26 October, and numerous users reported that code written in Java 1.6 (also known as Java 6) failed to execute. The problem apparently lies in an outdated version of the Java Virtual Machine (JVM), an application that allows OS X to interpret and run Java code. Java 1.6 was officially released in December 2006, but Leopard reportedly supports only Java 1.5.
It seems that you can't please everyone. Now some users are complaining that the new Java 6 is only for 64-bit Intel-based Macs only (it can't run on PowerPC-based or 32-bit Intel-based Mac) and it requires 10.5.2 (Leopard). More information in Apple knowledge base Article 307403 and on the Apple Java page.
I don't really use Java so I haven't installed it yet. Besides, I'm scared to install anything from System Update after the EFI update fiasco.
What about you? Do you use Java on your Mac?
Update: Reader Marc Chanliau writes "Actually, when you install the update, you can use Applications/Utilities/Java/Java Preferences and bump JDK 6 up the list. Then when you go to a terminal, you do see that the JDK invoked is JDK is 6. However, JDK 6 is not made the real default JDK (i.e., the "Current" symlink in JavaFrameworks still points to JDK 5). Since Apple recommends not to mess with symlinks, one has to wait until Apple releases another update to 10.5."