Cross-ontologies reasoning

The recent discussions on semantics and UDDI made me wonder about what happens when a query is formulated in terms of a particular ontology, but the registry contains information related to ontologies other than that one. I imagine some ontologies are so far apart (e.g. describing completely separate vertical industries) that reasoning across them would not make much sense. Other ontologies might be closer to each other, and including them in the same reasoning process could make more sense.

Does anybody know what is the current state of the art in cross-ontologies reasoning? Are there metrics that would allow us to determine how closely related two separate ontologies are and how much sense it would make to reason across them?

Thank you,
Ugo