I'm not a lawyer or an expert on the CDDL, however it could be that the license incompatibility is one-way: maybe only the GPL rightsholders have reason to sue.
Unfortunately, Oracle also holds copyright in Linux, so they can sue as a GPL rightsholder.
The real question is whether Oracle could convince a jury. It is hard to imagine what their argument would be. Then again, I would have never imagined that they would use copyright law over APIs to attack Google's use of Java.
