So, according to the last big news, the big question is no longer if, but how. This is how:
The closest collaboration point between free software Java developers outside Sun and the engineers on the other side would be the test suites. Since the free software projects like GNU Classpath and Apache Harmony, just like Sun's developers consider compatibility to be one of their prime goals, a merge of the respective test suite code to a common, free software, compatibility & regression test suite for the core Java specifications would be pretty useful for all implementors, as well as distributions using the DLJ.
Meanwhile, the simplest thing Sun could do to make open source Java reality a little sooner, and create a collaboration effort, would be to modify the current "Read Only" JCK license to allow compilation and execution of Sun's official test suite by independent implementations.