Panthro - 2:16 pm on Jul 23, 2012 (gmt 0)
This may be a stretch, but does anyone think this could be some kind of a clue? (from the recent "unnatural links" warning). My emphasis added.
"If you received a message yesterday about unnatural links to your site, don’t panic. In the past, these messages were sent when we took action on a site as a whole. Yesterday, we took another step towards more transparency and began sending messages when we distrust some individual links to a site. While it’s possible for this to indicate potential spammy activity by the site, it can also have innocent reasons. For example, we may take this kind of targeted action to distrust hacked links pointing to an innocent site. The innocent site will get the message as we move towards more transparency, but it’s not necessarily something that you automatically need to worry about.
If we've taken more severe action on your site, you’ll likely notice a drop in search traffic, which you can see in the “Search queries” feature Webmaster Tools for example. As always, if you believe you have been affected by a manual spam action and your site no longer violates the Webmaster Guidelines, go ahead and file a reconsideration request. It’ll take some time for us to process the request, but you will receive a followup message confirming when we’ve processed it."
Some of the phenomena reported here seems to suggest that Google is able to limit traffic without a visible change in SERP rank. Maybe if you're getting throttled, it's some kind of a "more severe action".
Again, this is just a thought but thought I'd put it out there.