I could see the flames rising at the start of this thread, so thank you both for steering away from them.
Essentially we have a format war here, in which one or other format will win and the other will go extinct. It might be being fueled by altruism rather than capitalism, and that's brilliant, but VHS and Betamax are watching from the wings. I know sod all about either of them except what has been posted here, but I see that they're incredibly similar, but just different enough to be incompatible; and I see that they are both horribly difficult for the lay-editor to use. By that I mean that the discussion between "oh this one only requires us to put in two new attributes instead of three" misses the elephant in the room: *both* formats require us to whitelist and start filling our wikitext with the HTML tag that the most iconic piece of wikimarkup, the double brackets, have kept hidden for nine years. The reason we brought in that now-ubiquitous syntax hasn't changed: the damn thing was too difficult for the layman to understand and use.
We do, without a doubt, need to implement this metadata-capture in MediaWiki somehow, but we need to do it not only in a way that the majority of people can use and understand, but in a way which doesn't make wikitext even more complicated for everyone. If either syntax were enabled, yes it would end up at the bottom of a template stack, but a) that's not going to do anything to ensure that the tags aren't being misused elsewhere, and b) even the most careful implementation is going to manifest itself in article wikitext along the lines of ""{{person|John Smith}}, born {{birthdate|12 June 1987}}, was a {{occupation|football player}} for {{organisation|Puddlemere United}}"". Or something like that. If we encourage editors to go the whole hog on this, we might as well install SMW.
There seem to be two usecases for these systems. First, marking up the 'stuff' that MediaWiki serves: images, copyright links, author links, etc. That requires MW to be able to get hold of the raw data for, for instance, an image license; and that's begging for things like new magic words to put on the image description page, not for enabling either format directly in wikitext. The only reason to do *that*, is to support editors marking up *their own stuff*, and that's where we have problems.
I think that it would be foolish beyond belief to encourage editors to divert their volunteer time to implementing a system that could turn out to be totally anachronistic within two years; and while I think it's a laudable long-term goal I think it would thus be very silly to let editors insert *either* format directly into wikitext at this point, or for a good year to come. By far the top priority should be implementing structures by which MediaWiki can *collect* semantic data. If we implement a {{COPYRIGHT:...}} parser function, or a metadata form, or (as I've been musing over for a while) a Category-esque system that wasn't based on wikitext and so which could have a fine-grained permissions interface; we create a feature that is useful whatever happens in the metadata world. We can implement RDFa with that data, microdata, both, neither or something else entirely. We could certainly expose it through our own API. Whatever happens, editor work is not wasted.
TLDR version: jumping on either bandwagon is neither necessary nor sensible, and we should avoid getting drawn into the issue. Implementing either of the proposed methods in raw wikitext actively defeats one of the purposes of MediaWiki: to make it as easy for anyone to edit stuff. It would need to be carefully thought through, and there's no point putting that effort in until we know which format has come out on top. Adding metadata to MW's own stuff is much easier, but its groundwork should be format-independent.
in this world of economic crisis, £0.02 seems to go quite a long way :-D
--HM