My AdSense impressions were maybe 10-15% lower yesterday than they should have been according to my server logs, but I did see a PSA when I looked at one of my page. PSAs are rare occurrences on my site (at least in my geotargeted region), so maybe the ad servers were misbehaving at some point.
My effective CPM is also running slightly lower than the January average (after being much higher for a couple of days), but this could easily be the result of a 30% traffic increase since the Allegra update. On an editorially diverse site, some pages or directories will have noticeably higher (or lower) CPMs than others, and if a lower-CPM subdirectory gets a bigger traffic boost than a higher-CPM directory, overall CPM will suffer even as total revenue increases.
Affiliate sales are part of the equation, too: A page that gets high affiliate revenues may not do as well with AdSense because users are clicking on the affiliate links instead of the AdSense ads.
Bottom line: Any number of variables can influence AdSense revenues, so the best strategy may be to focus on content and let AdSense take care of itself.