While kiba and shiko are both straddle stances, and have similar general posture, there are details both in the static posture (spine alignment, and foot, knee and hip position, for example) and in the way you move into the stance which make them two distinct entities.
I compared both of them over at MAP forums here
and at dan djurdjevic's forum here
, so I'll just link the posts for the sake of efficiency.
Structural differences aside, movement and application is also different, since to move into kiba-dachi you must rotate your whole body towards your front (which makes it awesome for kneeing people in the hamstrings and them taking them down with waist rotation, for example), whereas in shiko you rotate the other way (to expand), which gives it stupidly high amounts of sideways strength, but less rotation potential than kiba, counter-balanced with sinking potential due to the leg posture. They are both more or less equally mobile, but where kiba is good for applying force on the spot (prime example, turn into kiba for soto uke, then yoko empi; or the previous knee and takedown example), I feel shiko dachi is better for applying force while on the move (oi zuki, particularly low to the hara, or irimi waza, are very strong from shiko dachi, as is "bridging" for tai sabaki. an equivalent takedown would involve making a scissor motion with your arm and waist against your thigh and hips, similar to Enoeda sensei's famous kuzushi technique).
Kokutsu dachi, in this respect, has similar biomechanics to shiko dachi, in that you keep your center of gravity more towards your belly, without straightening the lumbar spine too much, and the rotation when doing it is towards your back, since your feet rotate to the outside, with the main difference being where your weight falls towards (diagonally towards your back heel in kokutsu, straight down between your arches in shiko). This makes it so that most movements in shiko dachi can be reproduced in shotokan's kokutsu with little or no adaptation. In fact, odds are that if you stand in kokutsu-dachi, and rotate your feet (with accompanying shifting of your center of gravity), so that your "support polygon", so to speak, is a roughly equilateral triangle instead of a rectangled triangle.
Sorry for the thread drift, guys, I'm prone to wall-of-text-ing when explaining stuff :p