Doug Purdy demonstrated versioned serialization in his presentation "Indigo: Using XSD, CLR Types, and Serialization in Web Services".
Until now the BinaryFormatter was not version-aware, that's why we always had to implement the ISerializable interface to deal with versioning. The attribute [OptionallySerializable] may now help! This attribute can be applied to types that are added with later versions of the class, and may not be deserialized.
The name of this attribute may change before the release.