I agree with all of DoP's points except for one. In Photoshop CS4 Camera Raw, it IS possible (indeed it is very easy) to apply a graduated filter effect under software - it is AWESOME!
You can even apply several graduated effects to the one image. I cannot speak highly enough of CS4, and I think I've only scratched the surface of its capabilities.
Haven't checked, but I don't think this feature existed in earlier versions.