Aaron Wall at SEObook has put together a thought provoking post on current linking techniques, structure, and how Google reliance on trusted sources & domains is in jeopardy as those resouces become more and more abused.
Anything that has been greatly trusted has been abused. The difference between the current abuse and past abuse is that in the past it was typically smaller individuals screwing with Google. Now Google has become a large enough force that they are actually undermining many of the business models of the providers of the content they are relying on.
Going forward, especially as Google, small content providers, unlimited choice, and easier access to the web marginalize the business models of many of the sites Google currently trust those sites are going to rely on users to help foot the bill. Google will give some content providers a backdoor deal, but most will have to look to user interaction to add value. That user interaction will be spamville. Thus I think rather than just trusting core domain levels I think Google is going to have to reduce their weighting on domain trust and place more on how well the individual page is integrated into the site and integrated into the web as a whole.
If everything Google trusts gets abused (it eventually does) and they are currently trusting raw domain related trust too much (they are) it shouldn’t be surprising if their next move is to start getting even more selective with what they are willing to index or rank, and what links they will place weight on.