IntelliJ IDEA 10.0.1 has a bug with regard to package objects.
I have attached a very small project, which has a package one.two. In the directory one/two there are two source files: MyClass.scala and package.scala. The project compiles successfully.
Note that when you open the package object, IDEA doesn't know what MyClass is inside the package object - which is wrong, because MyClass is just a class in the same package.
I'm using Scala 2.8.1 on Windows 7 and Mac OS X.