Ryan King’s Evolutionary Revolution has a lot of good info about making the web as we know it now a semantic datasource. He mentions, among other things, “microformats” (which are themselves a really useful tool that I’d seen but not understood).
Ryan’s entry, along with the articles cited in his blogliography ;-), should give PHP developers good ideas on the kinds of things to support in their apps, possibly by making data from those apps automatically available in one of the various microformats. For example, there is the hCard format, a point-for-point XHTML representation of a text-based vCard. You can already parse vCards with Contact_Vcard_Parse, so rendering one from the generated array should be a relatively straightforward task. You could then take all your vCards, parse them from source text, and make a web-available resource from them.
Good post, Ryan; lots of food for thought.