Whenever someone asks this question Wikipedia is a great answer, their internal linking structure is the most effective one I've seen yet.
No doubt about that, that's why it is consistently among the tops for any Google search. It is by far the best, I can't think of any that is even close to Wikipedia on this area.
IMHO, Google algorithms unfairly rank Wikipedia in the top ten for most searches. I don't think they are a model for SEO, though they might be for internal navigation. I'd like to see Wikipedia articles rank as well as they do now, when the "Google Knol" articles materialize. I think Google will quickly throw Wikipedia in the trashcan once their own service gets some reception. I remember reading today, that Yahoo Finance ranks below Google Finance just because Google is trying to "fairly" promote its services as a web destination. The "fairly" distinction was made by some Google manager in the article I read on John Battelle's site.
I would definitely expect Knol to be ranked higher, this is Google's company and their aim was to replace Wiki, it would be a certainly for them to do it. No company would be so good as to help competitors defeat their own subsidiary.
Why isn't Wikipedia model for SEO? I'd just love to hear this one. Extensive Internal Linking Massive Content Huge Amount of Backlinks I fail to see where Wikipedia is lacking SEO wise, but hey that's just me.