Google PR and indexing

Google seems to have changed its listing criteria yet again. One possible theory being posited for this latest sea change is the growth of sales in text links. Whether this is true or not is really a matter of making an educated guess, rather than anything based on concrete facts. Why so? Well Google don’t make a habit of releasing their secrets
Some search phrases that I normally track are returning a very mixed bag of results, while others are returning a lot of “spam” URLs and little or no real content. The first set of results is bearable, however the second set is quite disturbing, especially as the engine seems to have completly dropped most of the genuine listings for the keywords.
In the light of the constant changes to the Googlebot and its listing criteria this kind of change is having a highly detrimental effect on some commercial sites.