tedster - 3:06 am on Dec 8, 2012 (gmt 0)
I have seen this phenomenon quite a few times over recent years. When I dig into the data, what I find is that the keywords that are ranking are not changing, but only which landing page is showing for that particular search.
If total traffic remains the same no matter which keywords are ranking at the same time, then that might point to a kind of traffic throttling. However, I have never actually seen that kind of thing, and what mechanism (or even purpose) Google would have for it is also not clear to me at all. The only time I've ever seen something like traffic throttling, it was accomplish by timed shutdowns of any search traffic.
But what I have seen (the same keywords still rank but with a different landing page) is something that Google seems to experiment with and fine tune at a very micro level, rather than a timed update for everyone.