SEO Differences Between Google and Bing. Cuil founder Tom Costello has published an interesting article (discussed at WebmasterWorld forums) sharing his findings on the differences between Google, Yahoo and Bing search engines.
While the whole article has plenty of smart observations and ideas and is definitely a recommended reading, what I found especially interesting were the differences in how Google and Bing use signals to evaluate the site relevance and quality. A second major question is whether Bing is using any new signals, beyond what are usually used in search engines. Search engines use traditional signals like title, URL, emphasis/heading, document length and number of occurrences on page to generate an on-page score.