The only thing to be cognizant of is when you are bridging two programs the bridge is only allowing them to resources, it is NOT converting the program being bridged into a module.

For this reason you will usually find that your theme for your main site will not show up on the program being bridged, in this case phpbb3.

Also, even though it's been mentioned before I'll mention it again, BACKUP EVERYTHING and if your smart, try a test install under a subdomain. Screwing up a test site only costs you a few hours, screwing up your main site can cost you everything.