Is there a contribution available for search engine friendly forum urls? I think if you get rid of the ? in url, it will help.
Well - firstly if you search for "site:www.simplemachines.org", you'll already notice around 5000 results - which is a pretty good search crawl IMHO (Half the topics on the board).
Anyway - if you are running Apache there is an option in Feature Settings to disable the '?'. Only do it if you are on Apache - IIS doesn't support it.