I upgraded the Scala plug-in to version 0.4.296 and immediately noticed that the constructors (companion object apply methods, actually) for Some, Left and Right were erroneously flagged as "Application does not take parameters".
I rebuilt the caches, but still no-go. When I rolled back to 0.4.289, everything worked again.
IDEA: IdeaX IU, #98.402
Scala plug-in 0.4.296
Java: 1.6.0_22 (x86_64)
OS: Mac OS X 10.6.4