I apologize for not answering your immediate question (I think it might be quite difficult to predict what dirty legal tricks someone "might" pull in some uncertain future), but is your project really all that dependent specifically on OpenJDK? Or (if not) are you concerned that there might be some event in the future that will make running your code on any JVM (OpenJDK, IBM JVM, Oracle JVM, etc) impossible?
I'm less concerned about JVM incompatibilities and more about licensing terms.
In my opinion, you only own your technology stack if you can build the build systems of your product on your machines. I can do that with OpenJDK, and with Debian's apt-sources it is a matter of two command lines and waiting for the build.
From a glance at IBMs Java site I don't really find anything about their licensing. The Oracle JVM isn't open-source, nor is its JDK.
Let's be honest: the impact of most (including my) project to live or die with what Oracle does with Java in the future is marginal, in comparison to much bigger unknowns. Still I think that the big Java shops out there must have a contingency plan in mind, and I wonder if anyone could hint at what's their plan.
