I ran into a problem getting JDeveloper 11g installed on my OSX 10.5 machine. Partway through the installer you must tell it where your JDK is, and when you do it reports "Error, Invalid Java home". It took me a little while to locate some documentation on the issue, so I'm posting here in the hope that it helps someone.

So without further ado, to install JDeveloper 11g on OSX:

Download

First, get the base install jar for all platforms. It's just under a gig, and does not include the JDK.

Configure your preferred Java version

Open the Java Preferences utility (Found in /Applications/Utilities), and ensure that Java SE 6 s at the top of both lists.

If you don't have Java 6 listed, you'll need to get the latest Java update from Apple for your OSX version.

Add a symlink to Java 6

This is the part that fixes the above-mentioned problem with selecting the JDK. Open a terminal window and run the following commands:
cd /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home/
sudo mkdir -p jre/lib
cd jre/lib
sudo ln -s ../../../Classes/classes.jar rt.jar


Run the installer

For some reason on my machine, even after enabling Java 6 as my preferred version, double-clicking the Jdeveloper install jar would still fail (it runs but never goes anywhere), and and on the command line, 'java -version' was still reporting 1.5, not 1.6. So I executed the jar manually on the command line like so (one line):
/System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home/bin/java -jar ~/Downloads/jdevstudio11111install.jar

This brings up the installer. Proceed until you get to the JDK Selection screen. There will be no entries under Local JDK, so click Browse and navigate to /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home. The installer should accept this and you should be able to proceed with the installation.

That's it.

Note that Oracle provides a "JDeveloper.app" in the folder you installed to, to make starting JDeveloper easy. I created an alias to this in my applications folder.