The impact of doing morning cardio is probably insignificant for most people. However, some evidence seems to suggest that there might be some benefit to morning cardio if you are already quite lean and trying to lose those last stubborn pockets of fat.
I agree completely; I've seen some stuff to that effect. The whole morning cardio thing only becomes an issue if one is already quite lean. Then some dietary and cardio manipulation is necessary to shed the last bit, but otherwise it's just calories in vs. calories out. And HIIT burns more calories.
-casual