We could use this, but I don't see a big advantage over raw microdata if a) we'll be outputting as microdata at first anyway, and b) it's only expected to be used for a very few things like licenses, presumably hidden away behind templates. If it is done, though, it should be with curly braces for sanity's sake: {{#prop:author|Bob Smith}} or whatnot.
Who's to say it would only be used for licenses? I understand that this is the immediate want for microdata (or RDFa), but that want could expand at any time, and could expand greatly. Also, Wikimedia may only want to use it for licensing, but third party sites may choose to use it for far more than that.
Why shouldn't we use a technology neutral input format? What happens if microdata is replaced by something better/easier/simpler? I also don't necessarily think we should lock users into a certain technology. If we choose a nuetral input format, users can decide which output they wish to use (via extensions).
This sort of thing might be good syntax for a separate RDF stream, but I think we can keep that simpler. Instead of having {{Infobox foo|name=Bob Smith}} contain, somewhere, {{#prop:name|{{{name}}}}}, creating the triple (page name, 'name', 'Bob Smith') for the page, why not just leave out the #prop and have *every* template parameter create a triple? So {{foo|bar=baz|quuz=quuuz}} would create the triples (page name, 'foo|bar', 'baz'), (page name, 'foo|quuz', 'quuuz') with no extra markup needed. The triples could then be transformed into a more useful form by the consumer, using a language like OWL. This is something like how dbpedia.org works right now, AFAICT.
It is nice to be able to define an ontology separate from template parameters. In this scheme, every ontology that shares properties has to define the same template parameters. I think there are likely a lot of Wikipedia templates that share common properties, but do not use common template parameters. I for sure know my local wiki has templates with different parameters that use the same properties.
This also means that templates would create dependencies on one another, since parameters need to stay the same to keep the same properties; this would be hell on those who maintain templates.
Respectfully,
Ryan Lane