drall - Thinking out loud here.... are the changes you're seeing for something like very general queries that Google might be trying to disambiguate? I'm asking because disambiguation of some sort could be behind the "splintered serps", for want of a better description, that Google seems to be serving.
For those experiencing the changes, how would you describe the queries affected? Are they short and general, long and specific, possibly geo-related or possibly not, or for terms with ambiguous meanings? Anything going on that might indicate that Google is trying to determine searcher intent or to calibrate an interface?
Also, have you noticed any changes in auto-complete suggestions that might accompany these shifts? I'm wanting to get at some cause and effect here... at the least to get clearer descriptions of the kinds of queries affected and of any concurrent changes you're seeing.