Please note that the PHP 5.3 series will enter an end of life cycle and receive only critical fixes as of March 2013. All users of PHP are encouraged to upgrade to PHP 5.4.
Thanks for this notice.
I'll use that for any accounts that end up broken due to the switch, since it pushes the responsibility onto the script writers, and not the host running a secure server.
Remember that there are still a lot of people running old scripts of Wordpress and Joomla and other programs. Most don't upgrade.
I had to shut down the Joomla 1.0 sites on my server, and I am pushing the Joomla 1.5 sites to upgrade.
I actually can't believe how many Wordpress sites don't upgrade, when it is a simple click of a button.