In the EF forums, Danny Simmons lets an EF RTM cat out of the bag.

And, this looks like it has the possibility to be bigger than just for EF. The EF team worked with the WCF team to make ENTIRE GRAPHS serializable, and in an interoperable way. If you look at Ruurd Boeke's blog post,Circular references with WCF: solved a different way, about how he  achieved this, you can see examples of how WCF serialized entities before (non-interoperable) and after his own tweaks. Ruurd has been thinking about this problem for a long time, not just for EF, but for WCF in general, because it's the way that WCF serializes that causes the problem.

So the question is, did they just implement this specifically for EF, or will this have a broader impact? We'll see when the next CTP (as per Danny's comment, he says CTP not Beta4) of EF comes out.