You know, I'd forgotten that this wasn't implemented. It wasn't quite as straightforward to do this as the others when I wrote the Default Shader, and I must have forgotten to finish it up.
There a few ways you could work around this, but I can't think of anything that's really ideal. A safe bet is to mix between two copies of the Default Shader - one with specularity and one without. You would mix between them with the Merge Shader. Other options are probably more complicated, especially if you're using opacity maps.
Matt