Euphoria, the procedural animation system that's getting its big-time debut in GTA IV, is invisible.
Unlike the leap forward physics took when Havok landed in GTA IV, no-one coming to GTA IV is astounded by Euphoria.
And that's the point.
You don't notice when you drive up to pedestrians and they shimmy around the car. You don't notice that people pushed off of buildings fall headfirst, tumbling through the air. You don't notice that people stumble when you jab them in the kidney.
You don't notice because your brain doesn't register things that should be happening.
The best middleware is invisible.