These are the exact steps that worked for me.
Go to the apple dev java downloads page and get the latest one you can (the apple dev web site will not let you download packages that are unsuitable for your OS version).
I installed Java Developer Package for Mac OS X 10.6 Update 5 (Disk Image).
Then I executed these commands:
sudo ln -s /Library/Java/JavaVirtualMachines/1.6.0_26-b03-384.jdk/Contents/Home/src.jar /System/Library/Frameworks/JavaVM.framework/Home/src.jar
sudo ln -s /Library/Java/JavaVirtualMachines/1.6.0_26-b03-384.jdk/Contents/Home/docs.jar /System/Library/Frameworks/JavaVM.framework/Home/docs.jar
sudo ln -s /Library/Java/JavaVirtualMachines/1.6.0_26-b03-384.jdk/Contents/Home/appledocs.jar /System/Library/Frameworks/JavaVM.framework/Home/appledocs.jar
Then I restarted Eclipse (I didn't have to create a new JRE).
