Robert_Charlton - 8:31 pm on Feb 27, 2010 (gmt 0)
If the webmaster starts losing these valuable backlinks, it probably means he paid for them and decided it was not worth it to renew them.
I don't think that this dropping of backlinks, by itself, would be a dependable signal to Google. It might, eg, make google bowling easier, particularly if the bowler was buying the links... The bowler could drop the links as soon as he saw your rankings tank.
A lot of webmasters chase links from websites that have a decent PR which is stupid. No natural back link profile will consist of 100% PR3+ links. The fact is, you need relevant, quality back links regardless of the PR.
I agree with your comments as far as they go. Google looks at a range of quality signals. That said, while any site is susceptible to punishment by Google, particularly for obvious cheating of some kind, it is amazing what authority sites can do that other sites can't. In part this is because authority sites generally have relatively solid quality profiles.
Sites that rely too much on link buying are most susceptible to getting punished when bought links are discovered, but Google can make errors, particularly when a site is already in a borderline area.