Joomla already offers forum integration, if you could integrate Pligg into Joomla you'd kill two (or more) birds with one stone.
Joomla is possibly the biggest
CMS out there, a huge community base and lots of plugins...
I don't think Pligg really needs a forum anyway, the best idea would be to just release a style for phpBB that is the same as Pligg's, that way it will offer site consistency, unless you actually want to integrate the user base, which I think might be difficult unless you make Pligg an addon for phpBB.
Fact is in both cases, due to the support and developers other popular projects will always be ahead of this project, making it difficult to keep up with integrating latest versions, leaving many sites open to abuse.