It appears that the JCK licensing terms are holding an open source VM implementtion back - Apache Harmony.
Apache Harmony was started back in 2005 by a group of developers who were committed to implementing a JDK under the Apache License 2.0. Harmony has made good progress with heavy participation from Intel, IBM, and independent developers. If you look at the project roadmap, the plan was to start testing with the TCK (compatibility kit) right now (Q1 2007). Unfortunately, the Apache Software Foundation can’t agree to the licensing terms that the specification lead (Sun) licenses the Java Compatibility Kit under. Specifically, Sun has “field of use” restrictions in the JCK license that would add licensing restrictions to JDK and JVM released by the Harmony project. Here’s more explanation from the explanatory FAQ which accompanied the Open letter to Sun:
Q : What is a “field of use” restriction?
A : A “field of use” restriction is a restriction that limits how a user can use a given piece of software, either directly or indirectly. To give a concrete example from the Sun / Apache dispute, if Apache accepted Sun’s terms, then users of a standard, tested build of Apache Harmony for Linux on a standard general purpose x86-based computer (for example, a Dell desktop) would be prevented from freely using that software and that hardware in any application where the computer was placed in an enclosed cabinet, like an information kiosk at a shopping mall, or an X-ray machine at an airport.