I got a new iMac, and I have activated the first time usage of JDK6. It's all working, but it doesn't come with src.zip! I googled around but other's solution didn't work for me. (eg: http://stackoverflow.com/questions/4011002/java-eclipse-on-macosx-where-is-the-src-zip). I tried download their Apple JDK updates, but it still doesn't have the src.zip.
So finally realized that this is easier than it needs to be. You can download the full JDK source here:
After unzip, you should see the src folder under like this:
I've set this in my Eclipse's classes.jar as source folder and it works great.
PS: If you install the JDK7 from Oracle for MacOSX, it does come with the src.zip properly. However if you use Eclipse IDE with maven that set target to 1.6, it still convenient to browse that version of the source.