Search engines seem to disregard forums whose URLs are not search engine friendly. I'm referring to dynamic URLs. Some forums have modules which can convert these to user friendly URLs. But there is one effective way of doing it thru your web server's  MOD Rewrite directive in the case of Apache web server. In your case which one have you been using and why?

I think most search engines are okay with dynamic URLs now. However, it's the session ids that can stop the search engines from being able to crawl your site.

I think this is a hot issue regarding forums and SEOing is going to be very difficult thing in these days. We always have to work harder for this cause. Although there are many mods available for different forum setups but It is a tough job.