The big problem I can see with Semantic MediaWiki is that it involves horrible new wikitext syntax
Semantic MediaWiki lets you annotate links and values anywhere in wiki text with the property they denote. With SMW you put the property name before "::". That's it. Minimally horrible, compared to e.g. template syntax ;-).
So Berlin is the capital of [[Germany]] with a population of 3,391,407.
becomes Berlin is the capital of [[Is capital of::Germany]] with a population of [[Population::3,391,407]].
... although if that can be hidden inside the template code, all the better.
Yes, use of a templates may not have to change at all. Someone only has to modify the template to output SMW syntax and property names in wiki text.
Mostly what [infobox templates] need is standardisation
If all you're trying to do is capture information from infoboxes, DBpedia shows how much is possible already, and standardization can only help. But at some point you'll want to tell machines what the template field names represent. (DBpedia does an amazing job of inferring the type of a template field, but still gets some wrong.) The extensions that extend the SMW extension are evidence that its formalism for representing properties and datatypes within MediaWiki and thence in RDF is useful. I hope any effort to "bring semantics to infobox templates" would build on that.
Respectfully yours, -- =S Page