The poor quality traffic that we're seeing is related only to Panda returned sites in regions including the US and other than the US.
Much more work is required on those sites to bring them to the "quality" standard google is looking for. Google may be providing a "quality sandbox" to those sites providing quality results later as further improvements come through on those sites.
If further work is not done, we have seen sites slipping back under the threshold.
All this is somewhat theoretical observations - but that's my sense.