I wrote a blog post about this 2 years ago when they made the big cahnge.
http://www.oldwelshguy.co.uk/web-mar...osing-the-plot there is some info on there about how it is calculate.
There has been some new deleopments about this though and it is VERY disturbing.
test have been run (naturally), and it seems that it is site dependent! YES THAT'S RIGHT! Tests have been run on a new domain that was not ranking for the phrase, and google gave the quality score the thumbs down. The same page was then used as a landing page, same ad, same group etc, but to a TRUSTED domain that WAS ranking organically for the phrase, and the quality score was FINE!
Do no evil my arse!