Hmm.. how about we expose getRevision() as a public method instead? Then the callee can access it by grabbing the 'workflow' from the event (which will be this object) and calling getRevision() on it. I think that's a little cleaner / more general / more future-proof.

One thing which occurs to me is that you may be landing multiple revisions, especially in the future. So maybe a better implementation would be:

public function getRevisions() {
  return array($this->revision);
}

...so we don't back ourselves into a corner if we get better about letting arc land cascade through an entire branch or land multiple known revisions.