The shorter page idea was to reduce the bounce rates. Google and the rest of the engines are using engagement stats (bounce, # of pages, duration, etc.) as an indicator of page quality. That quality score is one of the ingredients in their ranking algorithm. Some would argue that quality is more important today than pagerank (link score). Quality score is hard to spam or buy, but pagerank is pretty easy to manipulate.
This ties directly into the Cutts comment about making a site that visitors like. Google has given away Analytics, and most sites have installed it, so they now have the data to easily compare engagement stats for different websites. I am not saying they necessarily use that data, but it is an option...
Since the permies forum is dynamic, we are working on some tweaks that will be made to code in templates, and viola, all the pages in the site/forum should be friendlier to the search engines. That should be a bump to the long-tail rankings.
I am a big believer in optimizing a navigation structure through a site for both SEO and usability. For example, "permaculture" is not a main forum category, even though one could argue that is the central theme for the website.
Also consider things like "growies" might be more friendly to search engines as "better-than-organic gardening" and "critters" might be better served as "chickens, ducks, goats, and other critters."
By working those words into your navigation structure, every page in the site gets a
boost for that added relevant content that reappears on every page. If you really want to rank for a keyword, you put it into your navigation and repeat that navigation on all pages.