I was one of the "originals" that caused the start of this thread. In light of the current discussion, I thought I would give you an update. Just the facts, because every theory presented here just gets shot down.
Old established US Ecom site.
I ran a decent sized adwords campaign for many years. I ramped it down then killed it due to non-performance a few months before Panda 1.0 hit me hard.
I've been infested by zombies and under strict throttling for over a year. Backdraft7 may remember that my zombie pattern seemed to run exactly inverse to his.
Aug 13: Started adwords back up for google product search. It worked pretty well so...
Oct 1: Started regular adwords back up. (Not as big as before, but big enough to test.) Results are mixed. I probably need to do a lot more tuning.
Nov 20: Zombies left and never came back. Throttling also seems gone. Both the quantity and type of visitors seem to be back to normal and smoothly random. Overall referrals seem to average about the same, but the daily and weekly counts now vary the way I would expect them to. The inter-day plots also look natural now, with the expected bumps at the expected times.
I stopped paying, and was hit by Panda several times then by zombies and throttling.
I started paying and the zombies and throttling were lifted.
Coincidence? You decide.