I've performed searches in the past and checked Google's cache of my pages, actually I'm doing that a lot lately to get a clue. In looking at one popular keyword phrase that used to bring me quite a bit of traffic, for which I no longer rank since the Mayday update, I noticed Google cache no longer highlights the full phrase.
Before: widgets for sale was highlighted.
After: widgets and sale are highlighted in different colors, for is ignored.
I don't have any instances of using the words widget sale without the "for" between them, could this combined with the change be why I lost all credit for the keyword phrase?
It's also possible this isn't new, I'm going from memory and am tired.