Not sure why but that .htaccess update broke my Drupal (v7) sites. The PHP class 'PDO' couldn't be found. I haven't the time to debug any further ATM, it may be peculiar to the rest of the .htaccess file. Fortunately my live site is small (the second is a test site) and I was able to do a quick test of 5.3 after commenting out the line that was added to my .htaccess file so I'm back up and running. Perhaps you can take this tidbit into account for future upgrades since the approach has the opposite of the desired result - i.e. keeping my on a 'functional' 5.2 install until I could test 5.3.