I've been watching each successive nightly release of the Scala plug-in to see that it exhibits a stack overflow error when editing a couple of files form the Scalaz project. (Specifically its Arrow.scala and Tree.scala source files.) Somewhere along the line of Scala plug-in nightly releases, Tree.scala stopped triggering the stack overflow, but Arrow.scala continued to cause it. This includes the current nightly build of the plug-in, 0.3.1188.
Now, with the IDEA 9.0.2 final relase (and plug-in 0.3.1188) the stack overflow no longer occurs.
Thank you, folks!