The spec is a little unclear regarding how unset properties must be represented, both during create/update actions (do they need to be listed?) as well as during retrieval.

Regarding retrieval... Section 2.2.1.2.1 seems to indicate that if a property is listed in the "Property Filter", then it must be returned. If it is not set, what value is returned?

For symmetry, we probably want the same representation for create/update as for retrieval.