From v3.2 on, self-hosted WordPress installs will require PHP 5.2. WordPress says that currently, only 11% of WordPress installs are running on servers using earlier versions than 5.2 of PHP, and only 6% currently run on MySQL 4 (after WP 3.1, minimum MySQL requirement will be version 5.0.15).
In a blog post about this, WordPress says:
“Many [WordPress installs currently on PHP 4] are on hosts who support PHP 5.2 — users merely need to change a setting in their hosting control panel to activate it. We believe that percentage will only go down over the rest of the year as hosting providers realize that to support the newest versions of WordPress…they’re going to have to pull the trigger.”