Bob,
William and Chris have both provided great insight on this. Review velocity has certainly come up as a possible trigger for filters, but so far, citation velocity has not been investigated or pointed out as an issue by anyone I know in the Local SEO community.
Does this mean it couldn't become an issue in future? I wonder about this. In the 10 years I've been working on the web, I have watched the rise and fall of directory submissions as a powerful tactic in traditional SEO. I am not sure if velocity had anything to do with that, actually, or if it was simply the Google decided that these types of links were not a great signal of relevance.
Obviously, right now, directory-type listings are the core tactic of Local SEM, and it's hard for me to imagine that changing any time soon, though Google appears to be trusting social signals more and more as relevance indicators. But as to the specifics of velocity, your guess is as good as anyone's as to whether Google will, at some point, create a filter that relates to this. We'll all have to wait and see.