Drupal 7 has support for the semantic web (RDF/RDFa). Is there a plugin or similar plans for WordPress?
As far as I know, there are no plans to implement RDF into WordPress. RDFa on the other hand can be quite easily implemented by any theme or plugin author by simply adding the RDFa attributes to the generated markup.
Writing a plugin to expose WordPress' taxonomy as RDF wouldn't be hard and making it two-ways would probably not be that difficult either. I know of no plugins in existence that does this, though.
Here's a start:
Probably wouldn't be that hard to write a plugin for it. What's your use-case?