Gregor Hagedorn, 04/04/2013 11:39:
If an editor saves a page with {{#property:population}} the parser looks up the current value and changes this to: {{#property:population|current value=2348732}} and stores this wikitext version in the Wikipedia. The same would apply to updating, saving {{#property:population|current value=2348732}} may result in {{#property:population|current value=2348700}} being saved.
A middle ground would be using the qualifiers for something like {{#property:president|as of=2013}} which would always transclude the president of /this/ mandate. However you'd still have to pass this parameter in any call for a template where the property is transcluded, while such stuff is often implicit. The problems Daniel describes are hardly resolved. (This said knowing nothing about the syntax, qualifiers etc. Sorry.)
Nemo