This is <a href="http://www.example.com" rel="nofollow">an example</a>.
... and is perfectly valid according to the W3C, the World Wide Web Consortium (who leave the content of the rel-attribute open, but do suggest to link to a meta-data profile). Other older values for the rel-attribute included were next, prev, contents, or glossary.
If this is indeed true, and the reason for this attribute is to fight spam, then it’s a step in the right direction but certainly no perfect solution. The idea of course is that as soon as backlinks become worthless to spammers in terms of PageRank & keyword boost, they’ll stop spamming comments, referrers, and trackbacks (assuming they spammed solely for SEs in the first place, and not for human visitors they might directly attract).
First of all spammers usually automate their tools, so for them it’s enough if they reach a small percentage of misconfigured blogs (those not using the “nofollow”). The rest of those supporting the new attribute will still have to live with the pollution, and delete their daily comment spam. This can be compared with an approach to battle email spam: “just don’t click on products advertised in those emails – surely this would make it an uncommercial business and end all spam.” Again, it only needs a small percentage of users to ruin this approach, and there’s always this small percentage.
But as the saying goes you shouldn’t throw out the baby with the bathwater: even if the “nofollow” attribute doesn’t solve all problems it might still be able to lower the amount of spam. Especially the one bigger systems get, because there are often central ways to configure these blogging packages, and spammers could learn to ignore these blogs.
Another, possibly bigger issue is that actually some links are valuable and should be followed by Google. Take Wikis for example, which often include very relevant pointers. But even comments can contain perfectly valid signatures. Most comment forms actually make it mandatory to provide a URL when you post; why would that be other than because it’s relevant? Do we want to have those pages be ignored in the future – and why do we display those links in the first place then, allowing visitors to follow them?
Update: yes, it’s officially confirmed now in the Google Blog by Google’s Matt Cutts and Jason Shellen (emphasis mine):
“If you’re a blogger (or a blog reader), you’re painfully familiar with people who try to raise their own websites’ search engine rankings by submitting linked blog comments like “Visit my discount pharmaceuticals site.” This is called comment spam, we don’t like it either, and we’ve been testing a new tag** that blocks it. 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. This isn’t a negative vote for the site where the comment was posted; it’s just a way to make sure that spammers get no benefit from abusing public areas like blog comments, trackbacks, and referrer lists. (...)
We’ve also discussed this issue with colleagues at our fellow search engines and would like to thank MSN Search and Yahoo! for supporting this initiative. (...)
We encourage you to use the rel="nofollow” attribute anywhere that users can add links by themselves, including within comments, trackbacks, and referrer lists.”
**Actually, it’s not a tag, but an attribute of an element, and Google introduced a new value to this attribute.
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