nope, not the same. if i take a photo with my 50mm lens at 1.4 its not the same bokeh or dof as if i took it with the d300. there are small differences in things like smoother colour transition, light to shadow transitions etc. small yes, but having shot the d300 i know when something looks a tad different. sometimes for the good, sometimes for the worse.
while in essense what you said is correct, its not quite as simple as saying that aps-c is simply a smaller image circle. for example, the pixel photosites are completely different sizes on aps-c you are squeezing 12mp's into a smaller area. on FX you are squeezing the same amount of MP's onto a larger surface area. so it would make sense that there are small variations in the way these photosites respond to light etc.
Alright thanks for the clarification although I'll be honest, I just don't see it, yet.