After an extremely long (IMO) release cycle the final version of PHP 5.2.0 was finally released yesterday morning. There are many new features, speed improvements and a fair number of security changes. You can read the official release announcement for a quick summary of the major changes and the specifics can be found in a very long and somewhat boring to read changelog

Link: PHP 5.2.0 is finally out! - iBlog - Ilia Alshanetsky