MacOS – Mac OS X 10.6 includes Java 6

The Java updates for OS X come from Apple, not Oracle (that "owns" Java and provides downloads for Windows and Linux). In general, Apple doesn't seem to support OS X versions prior to 10.6 (that's something like an unofficial policy, they seem to support the latest 2 versions only), so in general you can't get official Apple updates for anything, including Java, for 10.5 and below. There were ideas about some open source substitutes for Java, but I am not sure what happened to them. Given the recent Java issues, Apple may decide to create and distribute Java update for 10.5, but I won't bet on that.

JDK 7 will be installed under /Library/Java/JavaVirtualMachines/1.7.0.jdk, JDK 6 under /System/Library/Java/JavaVirtualMachines.

To trick OS X to accept Java 7 instead of proposing to install Java 6 a simple symlink is enough:

sudo mkdir /System/Library/Java/JavaVirtualMachines
sudo ln -s /Library/Java/JavaVirtualMachines/1.7.0.jdk /System/Library/Java/JavaVirtualMachines/1.6.0.jdk

Most Java Programms will run with this little hack without the need to install Java 6. OS X's Java Preferences (and maybe some others) will not as it seems to explicitly check the version of the JVM when it is started.