Regarding Richard's points:
1)Yes.
2)Search Engines are not born equal, some search engines are worse at finding content on sites. Google will follow links that have extensions such as .tcl. It does at least worry about query strings where a value of id is set, as it is possibly a session id.
3)For a site such as greenpeace.org, and possibly openacs.org, it is probably worth having pages dedicated to pointing search engines at the content you want seen on the site (a site map for search engines).
Tilmann:
Google has a spam reporting page.