Trust in Google
It would seem Google places quite a bit of ranking power behind "trusted" domains. For example, Matt Cutts, who works for Google, runs an internet marketing blog and decided to place a few keywords into the body text of a blog post having to do with buying cheap viagra. He then linked to the page with some identical anchor text and just a few days later was ranking in the top 5 for "Buy cheap viagra online".
This is interesting. I’ve had a similar effect with just one link pointing to a brand new web site, limufood.com, and I was number one for a particular search phrase: "limu food". Not that this is a competitive search phrase but it shows that Google puts high value behind the anchor text of a given link and will raise the bar for even a brand new domain. I purchased limufood.com recently and obviously is not a trusted domain (yet) but the combination of link anchor text and a domain’s age would seem to really have a major effect on rankings.