Well, I can tell you that it was created by google to help combat 'spamdexing', or posting your link on a site to use that sites search engine rank to influence yours. There are pros and cons to this of course as the 'nofollow' value is non-standard and not yet officially recognized by organizations like the W3C. You can find more info about it HERE
here is my little understanding on this subject. no follow and do follow. these are tag attributes introduced by google to tell spiders wether or not to give points to the site that is being linked to. basically, if a link is do follow, google credits that linked site for this backlink. if it is no follow, the linked site gets nothing for this.
Having just taken a quick look at the source code to this page, it appears as if though the "nofollow" tag is used for various intra-forum links, but with regard to signature file links, it is not used.
That is correct Nathan, a lot of forums don't use the nofollow (DP is one of them) attribute because this way they are giving an incentive to users to post more and more on them ...
I was always under the impression that you got credit from DP for your sig. links. i guess i was wrong and DP has "no follow".
sorry maybe it was not clear, but DP does not use "nofollow" and you do get link love from your signature links