As of 8-18-12, some critical components of Joomla 2.5 require a minimum PHP 5.3. For example, Akeeba backup's latest release will not run on PHP 5.2. I suspect that other components like admin tools will soon require PHP 5.3 as well. Any news about if/when PHP will be upgraded?
What could be done, as seen by other providers, to leave the choice of php version to the user. Those who need it can use 5.3, the others, for (in)compatibility reasons can stay at 5.2, until it is phased out.