phranque - 1:02 am on May 3, 2011 (gmt 0)
blocking a url with robots.txt is supposed to exclude the robot from crawling that url.
it doesn't prevent that url from appearing in the index, either as a url-only snippet or possibly with information obtained from sources other than the document at that url, such as the anchor text of an inbound link.
it is only about crawling, so there is no implied instruction for removal from the index.
also note that a robots.txt-excluded url that has a noindex meta robots tag or X-Robots header may possibly be indexed because the robots.txt exclusion prevent the noindex instruction from being requested.
the rel="nofollow" [google.com] anchor attribute is intended to prevent the transfer of PR and anchor text to the target url and will also drop the target url from the link graph unless the url is otherwise discovered such as through an xml sitemap or another anchor that is not nofollowed.