Yea iframe is the best way to go, I use it with phpbb3 and it works just fine. I bought a small mod on expertzzz to integrate the database with dolphin and works 80% fine. I just had to disable registrations in phpbb3 as new users registering in phpBB3 were not showing up in Dolphin contrary to what the mod author claimed. Not a train smash, new users in Dolphin are automatically added into phpbb3 which is what I needed. However be aware that you cannot integrate group functionality into phpbb3 so your Groups will still need Orca.
One more thing, if you stick to the phpBB3 default templates proSilver etc then iframes includes the entire forum page and your users wont have to scroll left and right to see whole posts. I am sure there is a fix for that but proSilver suits my needs so I didnt go down that road.