There is a standard already, called OPML that is already used in every RSS reader to store a list of feeds. Being XML behind, I'm sure it can be adapted to contain everything related to a bookmarking service; it's not a complete format like CalDAV and friends, but it looks like a solid base to build on.
OPML (I am familiar) could be used as a basis yep, but it's a file format which is more akin to vCard, vCal and iCal - CalDav, CardDav and WebDav are the transport(?) layers (API layers, operational layers, call them what you will) which sit on top of the file format standards providing the interoperability.
