The rel="nofollow" was introduced by the search engines back in 2004 (I believe). Since then we have not had a clear understanding on how exactly each search engine treats them. Some speculate that the rel="nofollow" means that the search engines won't crawl those links. Some say they will crawl them but they won't pass the link on as a vote for the web page it is linking to. Some say the link is completely hidden to search engines.
A DigitalPoint Forums thread shows that confusion in action.
One person said;
I just checked backlinks of one of my sites on yahoo site explorer and noted that the first site being shown in the results is a site which has my link with a "nofollow" attribute.
Does this mean that yahoo doesnt care about rel="nofollow"? Can anybody else check and confirm this from their own sites? And what about MSN?
Others noticed the same thing with Yahoo!
The response that seems to work for him is;
Technically, rel="nofollow" does not mean the search engines won't spider the page. They will follow the link, spider the page and count the link as a backlink. What rel="nofollow" means is "don't trust the link", i.e. don't pass PageRank/TrustRank, etc.
The robots meta tag "nofollow" is different, and really does mean "don't follow links from this page", and has nothing to do with backlinks or PageRank.
That works, but a clear definition from each search engine would be nice.
Forum discussion at DigitalPoint Forums.
There is also a thread from the Google's perspective that is currently active at WebmasterWorld.