The license is BSD based and can be found at http://framework.zend.com/license.
To contribute source code or documentation into the framework at any level (from a few lines, through a patch, to a full set of classes), you must first sign the Contributor License Agreement. This will also give you access to become a developer in the issue tracking system and the developer's wiki.
All framework code is covered by the Zend Framework Coding Standards. Learn them, love them, live them.
Development in the framework is backed by extensive unit testing. We use PHPUnit extensively as our testing framework. For code to be accepted, it must be tested and covered by a unit test.
The Zend Framework Issue Tracker is the place to submit issues of all types. This includes bugs, change requests, feature requests (if large enough, they could be submitted as a proposal), and small patches. It is also the place to track the release roadmap and view the current status of framework components.