Google's Matt Cutts talked more about Mayday at SMX Advanced a couple weeks ago. He said that it was designed to try and spot signals of quality on pages and sites that would be good for users, and that auto-generated pages and content farms tend to get hit the most by Mayday.
Barry Schwartz at Search Engine roundtable is pointing to a WebmasterWorld thread indicating that there may have been another tweak on June 23rd and 24th that had a big impact on some sites' rankings again.
Cutts' advice to webmasters affected by Mayday in the first place, was basically to improve quality. I'm going to go out on a limb and suggest that this advice would probably still apply.
Have you experienced a dramatic change in Google rankings this week? Let us know.