Pre-cursor: please skip if desired:
Like many other sites, my main site has suffered intermittent PR0 internal pages; being split into www & non-www (resulting in double listings), then being recombined again, etc. etc. My position is accordingly up and down like a ... yo-yo
My SERPs are changing daily. I was particularly concerned about the PR0s - but thanks to this forum it seems it is a common occurence with blameless sites at the present time. At least I haven't been entirely dropped from the listings like others - horrifying.
Google's Inbuilt problem:
It is PR! The slightest tweak to the Google Algorithm changes individual sites SERPS incrementally. But because of the inherited structure of the Google databases, a tiny tweak might cause very large effects across the whole structure (analogous to the 'butterfly effect').
So a confident move - like an attempt to filter out hidden text (and about time!) - might have enormous ramifications.
Consider it - what if the huge and high PR sites contain hidden text - how might their removal affect the backlinks of others? Worse, what about the even larger number of medium sized sites, cross-linked and linking to others - how might this affect the PR of milions of other sites?
I think this is why Google has moved from its indeterminate, and difficult to challenge, site penalty to the opportunity to repair apparent misdemeanors (spam) and be welcomed back into the fold.
Google depends upon large sites to maintain its structure and high relevancy. Large News and University sites, for example, must contain hundreds of pages unwittingly (or wittingly) breaking rules. The knock-on effect (Americans might prefer to call it a Domino effect) is largely unpredictable.
Nevertheles, I welcome Google's attempts to clear up searches, but it is bound to be painful. Some have questioned Google's motives in allowing GoogleGuy to contribute to the WebmasterWorld site, but some contributors have described it as a symbiotic relationship.
I suspect that it is indeed symbiotic - it makes sense in a database of hundred of millions of pages for Google to have the eyes and ears of people tracking search engine performance.