A DigitalPoint Forums post links to an interview Eric Enge had with Google's Search Evangelist, Adam Lasnik. In the interview, Adam discusses paid links, nofollow links, duplicate content and crawling, validated pages and how they are handled by the algorithm, and more.
Some interesting points he mentions are:
On buying links:
...our goal is not to catch one hundred percent of paid links. It's to try to address the egregious behavior of buying and selling the links that focus on the passing of PageRank
On duplicate content:
So, my core suggestion to webmasters would be to use Noindex and robots.txt to help us know what pages you'd prefer to not have indexed.
...in the context of duplicate content, penalties tend to be relatively rare. In the majority of cases it is innocent and unintentional. But, in cases where it's very extreme, there can be penalties applied.
The interview is extremely informative and I suggest that you give it a read.
Discussion continues at DigitalPoint Forums.