Yes, you will find a lot of open source (and some not) project that are based on aspects of WebObjects. WebObjects (WO from now on) is an old timer, dating from the mid-90's. It was originally created by NeXT, who had some very, very good OO minds. I find most of the design very beautiful, a sentiment that the J2EE APIs seldom inspire in me.
NeXT had it right, IMHO, a long, long time ago. However, it was not originally Java technology. It was originally written in Objective-C, a dynamically typed (yeah! no casting!) language similar to smalltalk but with a C syntax background. It was also about $50K for a deployment license. Only in the last couple of years has it migrated to Java - a move made for marketing, not technical reasons. Yes, its true -- I miss Objective-C.
OK, where am I going with this? The frameworks have a legacy of excellent design and they have been around long enough to be very mature. A lot of people have recognized the quality and "rightness" of the design and you can find implementations (some nearly complete, other just of parts) in Java, Ruby, and a number of other languages. TopLink for Java is heavily inspired by EOF (part of WO). How many re-implementations of EJB has anyone seen?
It seems that in the last few years that this has started to sink in elsewhere and there is movement in the Java world away from full-on J2EE to a simpler, saner world where we can concentrate on implementing functionality not infrastructure.
Why doesn't Apple market it? Who knows? I suspect they are having way too much fun selling iPods and raking in bales of cash. They have a great product, they know that. They use it for their own systems. Why expend effort trying to sell it in a market place choked with a myriad of OpenSource options when you can sell iPods and make much more money?
Chuck