Distilled, one of the more well-known and reputable SEO agencies have a good post on what they've been seeing: http://www.distilled.net/blog/seo/google-updates-april-2012/

I also work with a larger number of sites and have seen all kinds of crazy things in a number of different industries the last few weeks. Fllow SEOs at other agencies are seeing the same fluctuations.

I can confirm that this is happening. Usually I'll get a couple search terms on the first page that go up or down any given week. Most of the search terms I monitor have had lots of flux in the last few weeks.