Observation: I have found that one or two of our client sites that have dropped in rankings during the last 6 months no longer have "similar pages" listed. "Your search - related:www.example.com did not match any documents."
All those with good serps do have "similar pages"
The Sites that have dropped also had some of their out of date pages removed by us via the Google remove URL tool and the check box "remove outdated page" removes "anything associated with this URL"
Ray - EW
[edited by: ciml at 3:43 pm (utc) on Feb. 12, 2005] [edit reason] Examplified [/edit]