Dublin Core, RDFa, XFN, other microdata - what is your experience?

A blog post by @drummond (Daniel from PageProgressive) had me looking around to see what else might be on the horizon for us.

The basic concept is that web browsers don't see a hyperlink to a profile any different that a hyperlink to a map of Helsinki. By using additional attributes in our HTML tags, we can begin to teach the web browsers (and search engine bots) the differences and relationships between elements on a page. In the past, indexing was done based on content, not on markup. This might change things - for better or worse.

Initially, it looks quite interesting and something that could be helpful, not just for people trying to share a common set of data, but also for search engines and people performing searches. But, I can also see it going the way of meta keywords - subject to misuse and the relevance being reduced to zero.

What do you think? It would have a major impact on how we further our development - HTMLPurifier, TinyMCE, templates, themes all have their fingers in the markup. And what about content added by the site managers?