I had the same thing occur - same day even - the SERPs updated at 10:30pm EST on Saturday, really killed my night out. I have permission to use the trademark of my merchant and the site has been ranking either #1 or in the top five since the first of the year. The site has over 100 pages indexed by Google and good content is continually being added to it.
After this issue, it dropped to page 6. The site was a PR6 and dropped to a PR5. We had almost all our PR7 sites drop to PR6. Seems Google is "raising the bar".
What we have done is the following:
- updated to the latest version of WordPress
- fixed all 404s (even the ones that are stupid)
- Updated Title Tag, Description and added new content to home page
- Modified all Titles on site to be specific, accurate and compelling
- added 25 pages of new content (new content was spidered in less than 24 hours)
- scheduled a press release
- we are currently doing a full link audit
- checking internal linking issues
Any other ideas? We beat all competitors in terms of design, quality and content. Why we tripped a filter, I have no idea. We have been building our sites the same way for the last three years and this is our first penalty.