The requirements have changed as of WordPress 3.2. The minimum requirements for WordPress 3.1 are PHP 4.3 and MySQL 4.1.2.
That’s really it. We recommend Apache or Nginx as the most robust and featureful server for running WordPress, but any server that supports PHP and MySQL will do. That said, we can’t test every possible environment and each of the hosts on our hosting page supports the above and more with no problems.
Not required, but recommended for better security
Hosting is more secure when PHP applications, like WordPress, are run using your account’s username instead of the server’s default shared username. The most common way nowadays for hosting companies to do this is using suPHP. Just ask your potential host if they run suPHP or something similar.