There are several folders/pages that should be disallowed. I just created for our members here a robots.txt that can be implemented right away for Wordpress without problems. And it will help you to get rid of most, if not all of pages found in Googles the supplemental results, which cause a harmful leak of PR.

User-agent: Googlebot
Disallow: /*?*
Disallow: /*?
Disallow: /*.php$
Disallow: /*.js$
Disallow: /*.inc$
Disallow: /*.css$
Disallow: /*.gz$
Disallow: /*.wmv$
Disallow: /*.tar$
Disallow: /*.tgz$
Disallow: /*.cgi$
Disallow: /*.xhtml$
Disallow: /category/*/*
Disallow: */comments
Disallow: /feed/$
Disallow: /*/feed/$
Disallow: /*/*/feed/$
Disallow: /*/*/*/feed/$
Disallow: /*/feed/rss/$
Disallow: /*/*/feed/rss/$
Disallow: /*/*/*/feed/rss/$
Disallow: */trackback
Disallow: /*/trackback/$
Disallow: /*/*/trackback/$
Disallow: /*/*/*/trackback/$
Allow: /wp-content/uploads

User-agent: *
Disallow: /cgi-bin
Disallow: /wp-
Disallow: /wp-admin
Disallow: /wp-includes
Disallow: /wp-register.php
Disallow: /wp-login.php
Disallow: /wp-content/plugins
Disallow: /wp-content/cache
Disallow: /wp-content/themes
Disallow: /feed/
Disallow: /trackback/
Disallow: /rss/
Disallow: /comments/feed/
Disallow: /page/
Disallow: /date/
Disallow: /comments/
Disallow: /search
Disallow: /stats/
Disallow: /contact/
Disallow: /about/
Disallow: /archives/
Disallow: /dh_

There are probably more folders or files that might need to be disallowed. For example as you mentioned, the archives.

I hope this is useful for you and others here.

__________________
"Being an expert isn't telling other people what you know. It's understanding what questions to ask, and flexibly applying your knowledge to the specific situation at hand. Being an expert means providing sensible, highly contextual direction." Jeff Atwood
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