Linking over to a SEL post, Secrets Of Google Quality Score Revealed!!! (Not), which offers factoids and musing on Google's Quality Score ("QS"). An excerpt:
Myth: There is only one QS. Nope. One QS sets min bid, a different QS sets rank. Different QS for search and content.
Myth: Match type impacts QS. Nope. QS calculated only from queries which exactly match keyword.
Myth: QS improves with higher position on the page. Nope. QS is normalized to account for higher CTRs higher on page.
Full post at Search Engine Land.