A new video by Google’s Matt Cutts may give us some insights into how Google may treat links from different segments of sites, differently. In the video, Matt explains that Google doesn’t necessarily look to valuing a link from a blog more than a link from a forum. However, Matt does seem to imply that the trust factors they have in the algorithm might appear to SEOs that Google is valuing links from one group of sites as higher than another group of sites.
I am not sure if I explained that so clearly, take a look at the video and then re-read what I wrote and hopefully it will make sense to you. Here is the video: