If you are just looking to eliminate most of the duplicate urls that phpbb3 generates, you can just change your robots.txt file. http://www.branica.com/forums/websites/411-phpbb3-search-engine-optimization.html
phpBB3 can, in fact be SEO'ed. There's a mod out there somewhere that will do this for you, just follow the instructions. Using SMF for SEO isn't going to cut it. SMF is good, in fact is't sgreat, but it is no bettter SEO'ed than phpBB3 (or 2)