Vista - July CTP [datamember] changes?

I'm experiencing this problem with the July CTP. The following used to work
with the previous CTP.

[DataContract]
Public ref class baseClass
{
[DataMember]
int x;
}

[DataContract]
Public ref class derivedClass : public baseClass
{
[DataMember]
int y;
}

[DataContract]
Public ref class MyClass
{
[DataMember]
baseClass^ z;
}

The serializer seems to choke if the MyClass::z references an instance of
derivedClass. If I change MyClass::z from a type of baseClass to derived
class, it seems to work find. Was this change introduced in the July CTP?