Nice post, hvacdirect.
Bingo -- that is the factor that makes many feel this may be manually applied.
I hadn't specifically thought about Google's user data -- the clicks back to the SERP for a different choice. That could be one of many tip-offs. Now I doubt some of the affected sites I've looked at would suffer from such an affliction, but many of them certainly might.
The "get a good rank and then switch out the page" crowd is another factor I wasn't considering. It's certainly possible, especially since Google has an extremely long memory for historical changes these days. But again, I haven't spotted anything quite like this.