I am actually very curious about the effect of it... hence why I joined in the fun and then did a bit of research about it too. I am still not quite getting the nofollow links thingy. Care to explain it or best if you can blog about it here :D - I think it would be a great topic to discuss about.
Well, regarding the nofollow attribute in HTML anchor tag <a>, SearchEngineWatch has a very good article about the usage of nofollow.
However, since Kleio has requested, I shall write something about it from my humble point of view.
Why Comment Spamming Started?
No surprise, but practically every blogger is facing with the same issue - comment spamming. Why does it started in the first place? Is the site owner trying to promote his/her Viagra-like products to fellow blog readers? Not exactly.
Most of the comment spammers are doing that with hope to increase the Google PR of their sites, in a set of targeted keyword.
In fact, telling the truth (no flaming please), some legitimate bloggers are frequently dropping one-liner comment everywhere they could, hoping to achieve the same Google PR optimization as well.
As we write a blog comment, we will usually include our site URL if we have, and our name will be appeared as hyperlinked (such as Tien Soon says or Tien Soon said). Of course, readability wise, this linkage is of course a good one, so that everyone knows who we are. But, comment spammers were (Note that I use were, not are. I will why in the following paragraphs) abusing this convenience, not only to gain exposure from human readers, but search engine robots to be more specific.
rel="nofollow"
Due to the hectic problems as described above, Google has initiated to originate an idea of introducing a new relational description into anchor tag, which describes if a link is identified as legitimate or otherwise.
With this idea, all the major search engines came into the same stance to support a new anchor tag relation called nofollow.
For example, if we write a link to Google search site, we would normally write it as <a href="http://www.google.com">Google</a>.
However, if you would like to tell search engine robot, not to crawl to this link, but remain visually the same (still appears as a normal hyperlink) to human readers, then you would introduce the nofollow relation, such as this:
<a href="http://www.google.com" rel="nofollow">Google</a>
Blog Templates Are Using Nofollow in Commenter's Identity
With this nofollow relation, most of the blog templates are placing it into the commenter's identity. Thus, in most of the blogs you are coming across, the hyperlink to your site linkable from identity name in your comment, is not counted or contributable in whichever search engine optimization technique.
Some people who are aware of this nofollow thingy, especially the comment spammers, are quick to find an alternative. They actually include the links, as part of their comment message! This is a bit tricky to be handled, unless the blog owner chooses to globally apply nofollow in HTML header, and this will result in every link of the blog being non-crawlable.
Alternatively, the blog template can also be specifically scripted, to dynamically append rel="nofollow" in every anchor tag that appears in every comment. This can be done with JavaScript.
PPS is Using Nofollow
If you're a PPS user, note that the link of every ping is set as nofollow as well :) Thus, PPS brings you readers, but not exposure in search engine.