osCommerce V3 will need php V5.3
The osCommerce Team Leader, Harold Ponce de Leon, announced on the team blog that the current focus of the V3 project was to convert it to phpV5.3, a step he felt was very important for the future direction of osCommerce.
osCommerce V3 is a technical upgrade. The team have no plans to add to functionality of the core. According to Harold,
This long journey has never been about adding new features into the core to stop at and to possibly include a kitchen sink, but a long journey of improving a procedural based codebase and striving for a perfect, high quality level, framework to use as our foundation for future releases that can also be easily extended on.
phpV5.3 is not supported by many hosting environments at this stage but since there is no target release date for V3, that may not be an issue.
In the meantime osCommerce RC2a is still the way to go …