Simsi - 6:27 pm on May 3, 2010 (gmt 0)
I belive the reason the clean well seo'd sites are feeling the effects first is that these are the easiest ones for google to digest and update (onto caffine) but doing these first results in reduced positions until the final switch.
Expanding on Andylew's point a little, and noting that most people here at WebmasterWorld - SEO aware people - are reporting drops, is it not possible that Google's new infrastructure means that certain aspects of SEO are now less important, maybe even redundant, to Google?
Are the sites that are rising any different in terms of content to those that are dropping and is that content more "sticky"?
Ultimately, Google's aim must be to eradicate any ranking factor that can be manipulated. Sure that may take years, but... baby steps...?