@slinky101, @tedster, @Marvin Hlavac regarding forum sites:
I'm using a very UNcommon forum software, so definitely no recent software changes would affect me. However, the structure of pages is common for most forums: homepage has categories, each with summary of the most recent post, categories are paginated and are basically just lists of posts. Page 1 lists most recent posts. As posts age, they move to Page 2, 3 etc. Individual posts have links to other posts with related subjects. So, again, very common page/internal linking structure found on most forums.
In addition, most of the sites I've been able to analyze so far look exactly the same as they did in terms of number of incoming links (showed by Y!) , freshness of G cache for the homepage, number of Googlebot visits, searches for domain name etc.
Everything is the same BUT the traffic for most of the keywords that used to rank. Very long tail still brings some traffic but it actually looks like the situation deteriorates by the hour.
Hope someone can post more data about their forums so we can search for some common traits.