Why not letting OpenGL support something like this:
glHint(GL_SHADING_MODE_HINT,
GL_FASTEST // GOURAUD SHADIN
GL_NICEST // PHONG SHADING
Even allowing this with extensions.
A hint is just a hint, and an implementation is free to ignore it.
And concerning shading in general, there is nothing in the spec that says the default smooth shading is gouraud shading. It can be any type of shading, as long as it meets the requirements set by the spec.
For proper phong shading, you need vertex normals. You might just as well use some sort of custom lighting. With the current crop of fragment shaders and the stuff that is bound to come with GL2.0, it’s a little too late for specialized Phong shading support.