sun.reflect.NativeMethodAccessorImp.invoke0) and I've never been able to find the solution before.
Well, my google-fu must have been turned up to 11 today because I found this post detailing how to start the compilation server from within IDEA.
To save you reading the whole thread, the solution is just:
1. Open the 'Edit (Run/Debug) Configurations' dialog
2. Click on the [+]
3. Select 'Scala Compilation Server' (should be near the end of the list)
4. Give the new launcher a name (e.g. 'fsc')
5. Hit OK
6. Press the Play icon
Assuming you're also sensible enough to turn on the 'Use fsc (fast scalac)' option (it's under 'Scala Compiler' in the Project Settings), re-compilation of .scala files should get a LOT faster with fsc. I'm seeing single-file changes have gone down from over 10 seconds to less than one.