The Joomla! Project today announced that starting with Joomla 3.3 the minimum required PHP version will be raised to PHP 5.3.10.

This article explains:

• Why change now?
• Why wasn't the requirements bar set higher from the beginning of 3.x?
• How will extensions be affected?
• Will I get any warning? What do I need to look for?
• What will happen if my host is NOT Joomla! 3.3 Ready?

Read more at Raising the bar on security