I'm trying to figure out if I was hit with a penalty or the Mayday update as well.
May 4th 3:00pm EST (same day Google interface changed for me) my 5 main keyword phrases dropped from 1st position to 7th, 8th, 9th, 14th and 140th. My longtail traffic was effected as well. What's weird about the longtail traffic is I'm getting about 20% less now but it's totally different longtail phrases than before the May 4th changes. Overall traffic is down about 75% and continues to drop.
I have no other signs of receiving a penalty. Still rank for www.example.com, example.com, have 8 sitelinks, have 4 extra links under my main keyword phrase searches.
Googlebot went crazy on my website for a while but seems to have dropped below "normal" now. Google started indexing all kinds of pages I disallowed in my robots.txt but now seems to be correcting that. According to Webmaster tools there are 63 pages I disallowed still in the index out of the 1500 that were indexed.
In my niche it seems like the SERP's are very old at least 1yr except for 2-3 websites in the top 20. I check specific keywords in this niche several times per day for about 1.5 years now. The websites that are currently ranking in the top 20 with an exception of 2-3 are very similar if not the same as 1 year ago.
The SERPS have not changed much at all since May 4th for the keywords I monitor. It's as if they are stuck back in time, 1yr ago give or take. This is a highly competitive niche and keywords.
So I'm stuck between a new algorithm, old stale SERPs (waiting for reindex?) and/or I received a penalty of some sort.
[edited by: imbckagn at 6:17 pm (utc) on May 22, 2010]