Probably about time:
PHP 4 and MySQL 4 End of Life Announcement
First up, the announcement that developers really care about. WordPress 3.1, due in late 2010, will be the last version of WordPress to support PHP 4.
For WordPress 3.2, due in the first half of 2011, we will be raising the minimum required PHP version to 5.2. Why 5.2? Because that’s what the vast majority of WordPress users are using, and it offers substantial improvements over earlier PHP 5 releases. It is also the minimum PHP version that the Drupal and Joomla projects will be supporting in their next versions, both due out this year.