I have released a phpBB 3.X bridge for phpnuke, a demonstration site is available Here. You can find the files inside my forum Here.

Why a bridge instead of an integrated module ? In order to keep the integrity (the quality) of the excellent code provided by phpBB group with phpBB 3.X and in order to keep both part (phpnuke and phpBB 3) easily updatable, the bridge seems to be the best solution to me.