Add Virttual Folders

Virtual folders are containers that look like folders but are actually backed by a query or some other constraint. If an object is posted to this folder then the server creates an object containing metadata consistent with the Virtual Folder query, that is any object posted to a virtual folder will appear in the folder.
Similarly to the proposal for Saved Queries this facility provides for persisted navigational facilities.
Some questions:
• How would virtual folders fit into the existing Folder model (e.g. are they returned by getFolderParents)?
• Would an unfiled Document still be unfiled if in a virtual folder?
• How many repositories implement virtual folders?