The only way to get people to bug their hosts about upgrading PHP is to release popular software that requires it.. Without the demand to upgrade to higher versions, no host is going to bother.
My grief with this requirement is CentOS 5 only comes with php 5.1.6 without some hackery of special repos or self-compile... That affects a TON of hosting companies... especially with CentOS 6 still a few days away (which is a different challenge).
It was released in 2006, sure some hosting companies want to stay of the bleeding edge for fear of causing problems with the apps customers are running but if they haven't upgraded from that it's just laziness on their part.
i thought most hosting companies who still use centos do so because of cpanel or other control panel software that handles handles php very well itself.
Which ones don't?
Considering most hosting companies use cPanel and PHP/Apache updates aren't taken from the operating systems repository, this doesn't matter.
Use yum to list all 'php-' packages, unload them all and load in the 'php53' equivalents. I'm sure some cleaning up is involved but ...