You can find people on the phpbb.com boards to do mods real cheap. I got a 5 site discount but it was only 3.00 to get the SEO mod (per site) without mod rewrite which I don't think is necessary.
I removed session id and I have a html version of my forums (archieved) and rss feed. That works for me and get's me hits for around 1600 different keywords/month (avarage 10000 visitors/month from Google only)
edit: The rss feed is added only a week or two ago and rised the hits from Yahoo a little.
It's critical to remove the SID for Googlebot, et all.
Mod rewrite seems to help on the uptake some, and it's nice to actually see PR showing on each page when each thread is it's own .html file (or at least looks that way...)