ColourOfSpring - 5:30 pm on May 18, 2013 (gmt 0)
I hope Google isn't treating all guest posts as unnatural links, because that's pretty draconian. Guest posts can be of high quality, with an author's link to a site that your readers may be thrilled to discover.
My rule of thumb is this: if the article is good enough, it deserves to be published on a site that will earn the article's author a decent amount of organic click-through traffic to his/her site, therefore it shouldn't matter if a link is do-follow or no-follow (since they'll win targeted traffic), and therefore might as well be no-follow to be on the safe side. If the purpose of a guest post is primarily a link building exercise, then it's obviously an unnatural link.