In a post at Search Engine Roundtable, Barry Schwartz points out that Googler Adam Lasnick has been busy explaining the effects of the rel="nofollow" tag in some posts on the Webmaster Central group. In a post to the group on Tuesday, "Vanessa is confusing me ~ nofollow again...," he clarified that Google does not crawl nofollow links:
>Does Google crawl a rel="NOFOLLOW" tagged link and not give it credit,
>or does it just stop at the link and not visit that page unless it's
> found elsewhere?
As Aaron [Pratt] correctly noted, the answer is the latter :)
Google's handling of the tag was not made entirely clear when it was originally announced two years ago:
From now on, when Google sees the attribute (rel="nofollow") on hyperlinks, those links won't get any credit when we rank websites in our search results.
That same murky explanation is given in the Webmaster Help Center.
Lasnick and Matt Cutts have attempted several times to clarify exactly what Googlebot does with the tag.
Cutts stated explicitly that Google does not crawl nofollow links in July 2006, in his Bot Obedience: Herding Googlebot post: "At a link level, you can add a nofollow tag on the granularity of individual links to prevent Googlebot from crawling individual links (you could also make the link redirect through a page that is forbidden by robots.txt). Bear in mind that if other pages link to a url, Googlebot may find the url through those other paths."
Lasnick stepped in again last night to further clarify the issue in another post, If rel="nofollow" is becoming the norm. He notes that "nofollow links aren't listed any differently than other links in our Webmaster Tools backlinks section," and said that nofollow links will show up in search resulsts using the "link:" operator.
Matt Cutts has posted quite a bit on the rel="nofollow" tag. He's mentioned its purpose in stopping comment spammers May 2006, and talked about its use with paid links in a post from September 2005 called Text links and PageRank:
"The nofollow tag allows a site to add a link that abstains from being an editorial vote. Using nofollow is a safe way to buy links, because it's a machine-readable way to specify that a link doesn't have to be counted as a vote by a search engine."
The tag itself has drawn much criticism for its inability to curb blog spam, as it was originally intended. Most recently, Search Engine Journal's Loren Baker is calling for an end to the nofollow tag, giving "13 Reasons Why NoFollow Tags Suck," and David Wallace at SearchRank agrees in his post, "NoFollow Tag is a Dismal Failure."