Guest blogging is a very valuable component of not only SEO but social media as well. The beauty of this process is that it works both ways. The writer acquires some much needed relevant links and gets to build their resume and the blog acquires nice fresh new content. It is a win-win for both parties but you have to still be careful. You can still make mistakes and land yourself in the hot seat with Google.
In this Google video Matt Cutts discusses the ins and outs of guest blogging and how to take a quality approach.