1script - 6:17 pm on Sep 1, 2010 (gmt 0)
Any chance you can expand on this a little? Do you mean you've stopped writing new content targeting these keywords (i.e. using them in titles of new pages) or you've removed old pages that were optimized for those KWs. I'm assuming by optimization you mean the KWs were used in the title, h1 and a few times in the text as well as used as anchor text in a few links perhaps.
I recently stopped using a keyword group that had lower stats than others.
The additional info about your sample may provide some insight on whether new keywords are still being picked up / evaluated since many people here seem to have a feeling they are not.
The opposite of that would be that dropped KWs are replaced by other old KWs that were already in the index before MayDay (assuming this is MayDay's specific feature). That would mean the index is "frozen" and may probably move in the future once some sort of immense calculations are complete.