Proposal:
"The schemaVersion property is a relation between a CreativeWork (that typically contains structured data) and a specific version of a schema used in that document. The value can either be a version identifier string e.g. 1.2 or a URL to a (typically unchanging) snapshot of the document."

This is part of a larger proposal - see @danbri 's mail to public vocabs 2014-01-29.

This can be used by publishers to clarify the version of a (potentially changing) vocabulary. While it is designed for use with schema.org, it is also applicable to other "living standard" vocabularies such as Dublin Core and FOAF which evolve without changing their main namespace URI. URL valued properties are recommended in cases where a document uses multiple vocabularies, to avoid ambiguity.

For example,

    <div itemscope itemid="" itemtype="http://schema.org/CreativeWork">
        <meta itemprop="schemaVersion" content="http://schema.org/versions/1.92/"/>
    </div>

    <div itemscope itemtype="http://schema.org/Person">
      <span itemprop="name">Fred Bloggs</span>
    </div>

... could be used to indicate that the definitions from v.19