Just fyi, in the next release of mojoPortal we will have some seo improvements in the forums including a site map for forum threads that you can submit in google webmaster tools.
I think part of the reason our forums have done well in google is because this site has been around for a long time and gets quite a bit of traffic in general so google has done a pretty good job in crawling our forums. The new site map should help with newer sites to tell google how to crawl the forums.
Additionally we've also combined some of the url parameters so that the indexable thread urls only have 2 query string params since google recommends not having more than 2 params.
We've had the rel=previous and rel=next meta links for a while now but we've also created an all view for the thread which google prefers for indexing and we now use that for the canonical thread url.
This site is already running the latest code and I've submitted our forum site map last week, on this site that added around 10,000 urls that google is indexing for us, though it took a few days before they processed that new site map. I'm sure they were indexing some of those urls before but I definitely think these new improvements should help.