So I’ve just had to migrate a whole load of data to bbPress, and unfortunately the fairly old migration tool I was using (while generally excellent) didn’t bring across the topic-slugs, meaning I couldn’t have pretty permalinks in bbPress. So I’ve written a quick plugin which creates the topic slugs, it does this by doing 1000 at a time every time the page loads, then it very rudely bangs up a big text notice telling you it’s done and you should remove the plugin… at this point bbPress won’t work until you remove the plugin. You have been warned. I couldn’t be bothered with polite messages as this is a one use migration tool. 
Here it is: bbPress create topic slugs plugin – data migration tool only
Updated version, this one also create forum slugs: bbPress create forum and topic slugs plugin – data migration tool only
I was thinking i am lost but you saved me.
Very helpfull! You should consider add this plugin at http://bbpress.org/plugins/
Thank you very much for plugin.
thanks for the script simon. I test converted a forum from SMF (don’t even remember how, but I managed to do it through phpBB3) and it was missing the topic_slug and forum_slug columns.