Matt Cutts decided his next 30 day challenge would be to cause chaos and angst in the SEO community by announcing the demise of guest blogging as a tactic for getting backlinks.
It’s a lengthy post, with lots of justifications for the move, but boils down to this:
“So stick a fork in it: guest blogging is done; it’s just gotten too spammy. In general I wouldn’t recommend accepting a guest blog post unless you are willing to vouch for someone personally or know them well. Likewise, I wouldn’t recommend relying on guest posting, guest blogging sites, or guest blogging SEO as a linkbuilding strategy.”