hay. i'm a web developer working with the kohana v3 framework.
i have used various versions of phpBB over time as a solution for forms for web applications i make, and it's great.
after reading the RFC i understand why phpBB4 would probably use the Symfony2 framework.
if you want it to be fully php 5.3, have to use a php5.3 framework, and Symfony2 seems to fit this need ... i never worked with it and don't plan to ...
you can also consider the lithium (former Cake3)
it's also a php5.3 framework.
I think that the main problem of Web Application Development and a Form System is the user base of both.
if you want to archive real integration of a form system into a web application you developing you need both to have the SAME user base in both the application and phpBB.
so you can relate web application data items to phpBB users in an easy way. think this can give a huge boost to phpBB as a form system that can be integrated into different web applications and frameworks and give web developers and good form system to implant into community applications that are more then just forums.
so since phpBB4 will use the Symfony2 as it seems to me, best way to make such thing work is via strong and flexible API to the phpBB users system, so developers that use different web frameworks will be able to call API functions like: User Registration, User info, Delete Users and so on...