I have a couple of points to make on this, though I haven't actually run an OpenACS installation:
The extraction of emails by robots from an OpenACS installation should be small as only the maintainer has their email address available to view. User's email addresses are hidden until the visitors log in, which robots don't. I believe that the Directory package, is vulnerable to robots as it features a list of users and their email addresses.
Google will extract the data from a website to a certain depth dependent upon the PR of a website. My advice is to read many articles on indexing at somewhere like WebMasterWorld.
Much of the problems that Google has with dynamic sites is with things like session ids being stored in the URL (for example as "id=" with php). This makes it more reluctant to follow URLs that look like they have session variables in the URL (i.e. exactly "id=")
Greenpeace sound like they need better referencing for Google, pages primarily designed not for the user, but for Google to navigate to all content on the site.
