First of all, I wanted to thank to all phpBB contributors for the great job.
The reason I appeared here is that I'm very interested how things are going with developing phpBB4 on the Symfony 2.
When I heard about it first time, I was really surprised and nearly jumped on my chair. Great news!
I had some experience with building services, both large and small ones on Symfony (versions 1.1 to 1.4.8) and I can say I like it. The speed of development, modularity, automatization, generation of code/forms/database.
Also I did an very low-level integration of phpBB3 and Symfony 1.4.8, however, it still used two database connections.
Don't want to repeat all the argument used before, I just want to say it's a good time and chance for phpBB Team to use a framework building a new version of phpBB.