I just wanted to note for everyone's benefit that Scala Plugin Nightly Build 0.3.1814 works cleanly with IDEA EAP 95.413.
I use it experimentally as part of a enterprise Java project with eight modules with fairly involved depencies and over 500 KLines of Java code.
I have successfully built Scala classes that provide JUnit 4 test methods and extend Java classes. I have also successfully built Java classes that provide JUnit 4 test methods that extend Scala classes that extend Java classes. All in all thats a mouthful that indicates a solid product that delivers.
The IntelliJ IDEA Scala Plugin is an excellent piece of work. Alexander Podkhalyuzin and Ilya Sergey (and possibly others) have done fine work and deserve the thanks of both Scala, Java and IntelliJ communities. Thank you :)