Google's head of search spam Matt Cutts in his latest webmaster video has answered the question, "How can I guest blog without it appearing as if I paid for links"?
Cutts said that when his team reviews a spam report, a clear distinction between organic guest blog and those paying for links appears. Identifying specific differences between organic guest blogging content and spam, Cutts said that the spam content will not match the subject of the blog and will have keyword rich anchor text.
Matt Cutts said that a true blogger is someone who has subject matter expertise and doesn't use heavy amount of keywords in the anchor text. He also said that, "Guest blogging seems like it's the fad of the mouth". But, he suggests that the best guest blogging is when it is done in moderation & is not a full-time job.
At the end, Cutts said that if you are doing guest blogging to drive traffic to your website, you are not doing any good for your site's reputation.Matt Cutts Recommends Guest Blogging in Moderation!,