Hi Shawn,
I have noticed some of this lately with Google, and I do regular ranking checks over a wide range of keywords (using automated tool) for enough different sites to notice it as a pattern.
Google had been the most stable over the last few months (a lot of sites not moving much in either direction without big pushes), however lately it's been a bit more volitile.
I have seen however a LOT more volitility with Bing and Yahoo (which I mainly run SERP checks on for interest as it's not a lot of traffic). One site I've had jumps of 50 positions up to first page and then back down in Bing, and another that was front page in Yahoo for a heap of keywords and now not in first 10 pages.
BTW this has NOTHING to do with Google personalisation (which Timmy's results definately are) as I have it turned off in Google options and also clear cookies just to be sure before running my checks.