By your post and the subsequent links you posted, I'm not sure you understand the rel="nofollow". It's not relating to a meta tag; it will be an anchor tag attribute. For instance,

« a href="http://dont-wanna-link-here-.com" rel="nofollow" »

Yes, that's how I understood it, and that's what I mean with "meta" (I was not referring to the meta-tag in the head-section of HTML – possibly, this could be misunderstood). It's information about informatioon, describing the relation of the link. What is true for much other meta information is true for this attribute: people may "lie" about it (hide relevant information just because they oppose it), they are lazy (most people won't include it), it won't work universal (there will still be enough comments sections which won't support it).

It is my feeling Google does not favor these kind of solutions... nor do I think this particular approach is very useful. E.g. I wouldn't implement this approach for this forum because links posted here, 98% of the time, *are* relevant. Google and other search engines can do with them whatever they want, and they may increase the ranking of sites linked to.

By the way, Technorati introduced a similar approach recently:
<a rel="vote-against" href="http://mostevil.blogspot.com">I oppose Dr. Evil</a>
http://www.technorati.com/live/votes.html