Some of you may or may not know this, but there is a development forum for phpBB and it's located here -
http://area51.phpbb.com It's been open for years, originally used as a testing ground for phpBB 2 before that came out officially (yes, that's why I said years

). Just last week, the team have set up a
new forum there specifically for the discussion of the development of phpBB 4.
Codenamed
Rhea, named after one of Saturns Moons and following on with their planetary/space theme established by phpBB3's 'Olympus', the phpBB team are looking for a lot more community involvement with this and it's looking off to a good start

You can also check out the phpBB 4 wiki:
http://wiki.phpbb.com/PhpBB4