http://www.mattcutts.com/blog/guest-blogging/

"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." ~ Matt Cutts ~