Looking to rewrite urls for any forum is dangerous. The reason I say this is because the forum system is built to handle moving topics around and adding up to millions of posts. Trying to track all this is one thing, but then adding a url rewrite module on top of it is asking for some trouble.
As for search friendliness, Google is going to be more friendly towards the core phpBB3 urls and less friendly towards rewritten urls. It's true!
This is just the topic I was looking for. I was also interested in changing my url's but not so much for seo, but for security. I see why you recommend not changing urls and understand the dangers in doing so.
Does RocketTheme, or anyone else here on this forum for that matter, have some good tips for securing phpbb3? I'm doing a lot of work on my forum and also plan to invest in advertising for it, and would hate to see it hacked after all the effort I've put into it.
I have done plenty of google searches on the subject, but only seem to find outdated info for previous versions of phpbb.