The `eclipse` task (and I assume `idea` as well) requires at least one source file of the relevant type to be found within a source directory. For example, for project meta generation to succeed, the src/main/java/ directory (and/or subdirectories) must contain at least one .java file. The same holds for scala. If these source files are not specified, the task silently fails and just doesn't generate any project meta.
It seems to me that this would be a fairly common use-case: using Buildr to setup a project prior to creating any sources and then doing the actual coding within an IDE. I would propose that the source autodetection be fixed to just assume that there will be source files within the relevant source dirs if they exist. At the very least, a warning should be printed regarding the absence of sources and that this prevents IDE meta gen.