Eric Blue is looking for a standard format for mind maps:

In an upcoming post I'm going to go into a little more detail on what this format should look like, and what technology would be the best fit (XML, JSON, RDF, N3?). In the meantime, I'm eager to hear some feedback:

* Is the need for a new standard an accurate observation?

* What needs to take place in order to make this vision happen?

* Who needs to adopt this format first, and what will be the major motivators to make progress in this area?

Source: The Need for a Common Mind Map File Format

I would suggest checking out discussions concerning the intersection of topic maps and the semweb:
W3C: A Survey of RDF/Topic Maps Interoperability Proposals and Living with topic maps and RDF.

Also, I would delve into the approaches for internal representation and storage used by Mindraider and Pragmatron.

hey, use SKOS out of the box, thats great.

if not enough, checkout OpenIris.org

if not enough, checkout the pimo as suggested here:
http://www.dfki.uni-kl.de/~sauermann/2006/01-pimo-report/pimOntologyLanguageReport.html