I've started working on getting the development version of Drupal 7 installed and running so I can get familiar with the changes that are coming and work on having a d7 version of my Post Reminder module ready when d7 hits production release.
No sooner had I installed the new version than I got my first error
Fatal error: Class 'PDO' not found in .../includes/database/database.inc on line 137
Fortunately, it's a solved problem. A quick google search found the fix: adding two lines to the php.ini file to enable PDO
extension=pdo.so
extension=pdo_mysql.so