According to this new post on the Mambo Foundation's blog, they've made a decision on what to base the next version of their software on - the CakePHP PHP framework.

After a great deal of research the Mambo team has decided to utilize the CakePHP framework for Mambo 5. CakePHP is a rapidly evolving, mature, and feature rich PHP framework. The project is backed by an official Foundation (http://cakefoundation.org/) much like the Mambo project itself. We believe this is an important criterion as it helps assure the project will remain active and community minded.

They include an overview of some of the features of the framework they plan to use including their flexible license, the simplicity of the development process and several "hot features" like built-in validation, access control lists and flexible view caching.