The update was manually pushed and is still not running as a rolling update - so I was wrong on that front.
Webmasters believe this update began on July 22nd, which goes along with Google's statement of it being pushed out "late last week."
This update incorporates some new signals that help differentiate between higher - and lower-quality sites. As a result, some sites are ranking higher after this most recent update.
Past Google Panda Updates:
- Panda 2.2 on June 18th or so.
- Panda 2.1 on May 9th or so.
- Panda 2.0 on April 11th or so.
- Panda 1.0 on February 24th
For more on Panda see our Panda category.