“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.
In less exciting news, we are also going to be dropping support for MySQL 4 after WordPress 3.1. Fewer than 6 percent of WordPress users are running MySQL 4. The new required MySQL version for WordPress 3.2 will be 5.0.15.
WordPress users will not be able to upgrade to WordPress 3.2 if their hosting environment does not meet these requirements (the built-in updater will prevent it). In order to determine which versions your host provides, we’ve created the Health Check plugin. You can download it manually, or use this handy plugin installation tool I whipped up. Right now, Health Check will only tell you if you’re ready for WordPress 3.2. In a future release it will provide all sorts of useful information about your server and your WordPress install, so hang on to it!”
Full post: http://wordpress.org/news/2010/07/eol-for-php4-and-mysql4/