"The best preparation for good work tomorrow is to do good work today." (Elbert Hubbard)
The clock is ticking... Java 8 is not that far anymore. The Adopt OpenJDK initiative is to OpenJDK, the Java SE Reference Implementation, what the Adopt-a-JSR initiative was to Java EE 7 / GlassFish 4 a few months ago. The idea is the same, i.e. let people get familiar with the Reference Implementation and its different APIs before they are released. Let them play and stress test those implementations and provide as much feedback as possible before those implementations go final.
In short, Adopt OpenJDK allows the Java community to prepare the field for JDK8. There are many ways to contribute. The first step could be to test existing Java applications on top on OpenJDK 8. The next step would then be the adapt those existing applications to leverage some of the new Java 8 enhancements (e.g. Lambda), etc.
From a Java EE angle, this effort also makes a lot of sense. After all, 'we' all run on top of the JVM! So have you already tried to run your current Java EE environment on top of an early JDK8 build? What were the issues, if any? Have you tried to tweak your existing Java code to use some of the new Java SE 8 features? If so, please do share your findings with the community, the Adopt OpenJDK program is great venue for that.