You could go that route.  Although, there is a good chance that you would soon reach a point where you wanted to provide other secured resources or features to users of your forum.  You could have a public DNN site (no registration required) that fronted a phpBB (or any other) site through an IFrame.  Personally, I would not recommend the approach... but then again, everyone's use case is different.  Is there some come compelling feature in phpBB that you could not satisfy using the DNN Forum Module or ActiveForums, which would cause you to consider limiting the rest of your future site functionality as well as commanding dual sites and dual maintenance?  I am sure the Will over at ActiveModules or Chris on the DNN Forums team would be keen to hear your feedback.

Cheers

Scott Willhite, Co-Founder DNN