One of the many perks about visiting Cre8asite Forums, is that you can basically have a one on one conversation (visible to the public) with people who have a deep understanding of how search engines work from the inside. One such person, and I know he will hate me for saying so, is Ammon Johns, and in this thread named One-way links?, we got into the discussion of how search engines view the link map of the Web. I plan on writing a short article on the topic, with pictures to better explain the thread, but for now, I'll give a quick overview - the thread is worth checking out.
In the thread we discussed the idea of not linking from site a to site b, because it is very easy to detect. Ammon adds that even Pyramid / Triangular Linking Strategies are also easy to detect, if you think of how search engines look at the link map of the Web. Ammon explains that search engines look at the link structure of the Web from a 'six degrees of separation' type of manner. He then compares it to a graphical illustration of a traceroute, where you can see how the nodes of the Internet interconnect. If you can visualize a search engines link index and then map all the links between the sites, by grouping them into small communities. Use different colors to link major hubs and authorities to smaller sites. If you see an unusual level of linkage outside of a specific community, or even something that visually looks wrong, then you can find an unnatural linking scheme.
Take a look at the thread, it is not rocket science, but it is something that is often overlooked.