The only people really using rel=”nofollow” tags on both internal and external links on their website, or using them at all for that matter, are people who know a little bit about SEO. (package installations excluded)
Inherently google is looking for ways to identify sites which are trying to game the system, sites with unnatural backlinks, unnatural and duplicate content, etc.
The heavy use of the nofollow attribute across the web has devalued googles page rank algorithms as they currently stand. Sites such as wikipedia have added nofollow tags to all external links in order to reduce spamming by SEO’s, but in the process, wikipedias references to important sites carry no weight.
Call me daft, but I really don’t think google pays attention to the nofollow attribute on sites such as wikipedia. It is in the best interest of googles quest for the most relevant results, for those links to be given substantial weight.
Any coder with any chops, let alone one of the google elite, could write a script which compares the number of nofollow links, to the number of do-follow links and come up with a nice little ratio. Based on that ratio a fair assumption can be made, either this guy is heavy into SEO, or this guy doesn’t know much about SEO.
Since nofollow links devalue the google PR algorithm as a whole, if I were google, I would be VERY selective about paying attention to the rel=”nofollow” attribute.