I was thinking the same thing and started this topic while ago: http://www.ez.no/developer/ez_publish_3/forum/general/integrating_ez_with_ie_phpbb_suggestions
I have started to try integrating these systems together, and will inform when/if this project will work...
Regarding whether it's a good idea or not: Have you tried installing the forum site that comes with ezP 3.2? My thought is that if the eZ forum can fill your needs, or at least after some basic modifications, this would be the way to go. I was also looking to integrate phpBB with ezP at one point, but today, I think I'd rather build something else in ezP as it would be much more flexible.
