I believe it's a part of the new url parser; myself for one i like it on my site, as I dont' want to be held responsible for those users who link to sites that don't fit in with our site (such as those with questionable content)
What the heck has "nofollow" has to with that?
nofollow helps google and google alikes on pageranks, that's it. Nothing more, nothing less.
nofollow on sigs imho could be okay, but no real need. In posts, nofollow should be abscent, as posting a link on many forums should increase the pagerank of that url.