I don't think it's Panda for the May Day I saw. Why? Well Panda impacts main keywords and longtail. My main keywords rank the same (but less traffic in multiple niches). Longtail of course I don't track but lets look at a classic example.
I have Site A that received 500 different keywords a day to a URL.
Since May Day the # of different keywords is now around 250. The main $$ keywords are exactly the same rank.
This is consistent across all the sites I saw impacted.
So two theories I have:
a) Someone talked about the adsense pollution. This could be coupled to an adsense update that has more coverage of long tail keywords. Since these Keywords are rare search terms and now they have more Adsense coverage the advertiser is scraping these results away from organic.
b) Negative link velocity. This is the only thing I see consistent. Sites I did NOT build links to (just let google's awesomeoness of organic link building happen, haha) have had some negative link velocity.
Sites (I have 100s) I did build links to in last 30-60 days NO change in traffic.
I am not talking 1000's of lost links, just a dozen or so a week or two weeks tops but over the last few months that may add up to something.
I am again at 100% speculation but it's my best educated guess.