The next major release of phpBB, phpBB 3.1 "Ascraeus", will require PHP 5.3.2 or newer.
PHP 5.3 provides important functionality that a number of established projects are using. As it is phpBB's goal to use components from other projects where doing so makes sense, raising minimum PHP version to 5.3 allows phpBB to reuse some components from Symfony instead of reimplementing equivalent functionality. This will speed up the completion of phpBB 3.1 by reducing the amount of work that Development Team has to do.
You can read the discussion
leading to the decision.
All major distributions are already shipping PHP 5.3.2 or newer in their current stable versions. You can find a table of several distributions and the versions they ship here
phpBB 3.0 "Olympus" will continue to require PHP 4.3.3 as the minimum version.
phpBB 3.0.10 supports PHP 5.3. Since PHP 5.2 is no longer maintained by PHP developers, all users not already running PHP 5.3 are strongly encouraged to move to PHP 5.3.
As phpBB 3.1 "Ascraeus" is still under active development, there is still plenty of time 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 5.3. Instead, sections of code will be updated as needed in the 3.x release cycle.
Please discuss this announcement here