You've probably read that Mac OS 10.6 has finally made Java 6 the default, and you may also have read that there are both 32-bit and 64-bit JVMs. (That latter part will only seem surprising to you if you'd been using Java 6 on 10.5.)
What I didn't know until I read Things Removed in Snow Leopard was that Java 1.4 and Java 5 have actually been removed from 10.5. (Cocoa-Java has been removed too, but we were told to stop using that years ago, and we know that what Apple deprecates in one version, Apple tends to remove in the next.)
If you've been paying attention to Ubuntu 9.10 – which I hadn't, but someone else pointed this out – a quick sun-java5-jre package search shows that while Ubuntu 9.04 ("jaunty") offered the sun-java5-jre package, 9.10 ("karmic") doesn't. Both have Java 6 though: compare the sun-java6-jre package search.
In the Ubuntu case, you'll still be able to install a Java 5 JDK/JRE downloaded direct from Sun, but in terms of ease (and likely default), it looks like we're finally entering a Java 6 world.
The people least likely to be happy about this are those running Java applications on a Unix box, displaying on a Cygwin X11 server. Sun's Java 6 and Cygwin's X11 server really don't get on well.
Mac OS 10.4 users might not be too happy as apps start requiring Java 6 since for them that means an OS upgrade: there are no practical third-party JVMs they can use. And for some, an OS upgrade will entail a hardware upgrade because 10.6 is the first version that doesn't support PowerPC. But now these users are two major OS releases behind, Apple's "deprecate one, remove one" tempo means lots of stuff's going to start breaking for them anyway.
I'll be curious to see what happens to the Omni Software Update Statistics over the next couple of months. For 10.5 to have only overtaken 10.4 in 2009-02 is quite shocking for a group so traditionally upgrade-rabid as Mac users. (Though these numbers aren't necessarily representative of all Mac users.)