After much internal discussion, the Development Team has decided that phpBB 3.0 "Olympus" will be the last in the line of phpBB releases that will officially support PHP versions less than 5.2.0. All future feature releases (and their maintenance releases), beginning with phpBB 3.1 "Ascraeus", will require PHP 5.2.0 or greater.
Following the release of phpBB 3.1 "Ascraeus", phpBB 3.0 "Olympus" will remain supported for a limited period of 6-9 months (further information on this will be released as it is decided upon) -- this version will remain compatible with PHP4.
As phpBB 3.1 "Ascraeus" is still in early stages of development, no release date has been set. Consequently, there is still plenty of time for you to upgrade your PHP installation.
Additionally, one should note that this announcement does not imply that phpBB 3.1 will see large portions of code rewritten in order to take advantage of new features in PHP 5.2. Instead, sections of code will be updated as needed in the 3.x release cycle.