With the latest EAP of Idea 12, Use External Build is enabled by default.
This doesn't work for me and I presume isn't supported by the scala plugin. Disabling this option and everything works fine again.
One thing that caught my eye was the option in the external build to compile independent modules in parallel. This would be quite useful as it could speed up a full build considerably for my project.
Have you got any plans to support the external build in the near future?