Researchers at Stanford have produced a paper on search engine spam or as they define it web spam. You can read the paper here... Some extracts raised my eyebrows such as...
"... Please note that according to our definition, all types of actions intended to boost ranking, without improving the true value of a page, are considered spamming."
Does that mean removing session ids or changing a company name in the title tag to include a keyword is spam by their definition?
There are various techniques left out (hopefully intentionally as it is a spammers guide as it is). Their suggestions for search engines to better their algorithm make sense. I believe however Google already implements at least two of their suggestions.