I've written before about the idea of typed citations.
Mark Liberman (who I might have studied under if I'd gone ahead with my PhD application to UPenn) comments on the idea of typed citations with some excellent thoughts. One thing that I realised, reading Mark's post: I probably wasn't clear that I was envisaging a controlled vocabulary, much like XFN has.
The notion of typed citations relates to trackbacks, a topic I've also talked about before. Bryan Lawrence (who has recently become my main sounding board in the development of Leonardo) asks about semantics in trackbacks. He is talking about typing the source object rather than relationship but the two are related. In RDF terms, one is a class, the other is a property. I would love to see both able to be expressed in a trackback.