directly from google webmaster central blog:
what does violate Google’s guidelines on link schemes is when the main intent is to build links in a large-scale way back to the author’s site. Below are factors that, when taken to an extreme, can indicate when an article is in violation of these guidelines:
* Stuffing keyword-rich links to your site in your articles
* Having the articles published across many different sites; alternatively, having a large number of articles on a few large, different sites
* Using or hiring article writers that aren’t knowledgeable about the topics they’re writing on
* Using the same or similar content across these articles; alternatively, duplicating the full content of articles found on your own site (in which case use of rel=”canonical”, in addition to rel=”nofollow”, is advised)
Sites accepting and publishing such articles should carefully vet them, asking questions like: Do I know this person? Does this person’s message fit with my site’s audience? Does the article contain useful content? If there are links of questionable intent in the article, has the author used rel=”nofollow” on them?