I'm running CentOS 5.5 here with Apache2, PHP5.3.3, and MySQL 5 (Ver 14.12 Distrib 5.0.77, for redhat-linux-gnu (i686) using readline 5.1). My wordpress install on the same system runs perfectly, but the drupal7 install script only detects SQLite. The mysql module is enabled in php.ini, so that isn't the problem. Do you think it could be something with Drupal 7, or my PHP install? I tested it on localhost (I'm essentially running ArchLinux with Apache) and it installs just fine. I don't see a difference between my local php.ini and my server php.ini. I get this when accessing install.php on the server.
SQLite The type of database your Drupal data will be stored in. Your PHP configuration only supports a single database type, so it has been automatically selected.
Edit: The mysql PDO module is installed already.
Final edit: The issue was that PHP wasn't actually loading the modules. It was looking at the file in /usr/local/lib/php.ini rather than /etc/php.ini. After implementing the suggestions here, I changed the php.ini file configuration, restarted apache, and it works fine. Thanks to all who replied.