You might want to take a look at the RigidbodyFPSWalker script on the Unity Wiki.

directly controling Velocity ;)

var speed:float=4;
function FixedUpdate(){
    rigidbody.velocity=(Input.GetAxis("Horizontal")*transform.right+Input.GetAxis("Vertical")*transform.forward).normalized*speed;
}

It may not be the Character Controller see

http://forum.unity3d.com/viewtopic.php?t=39946

you can have constant velocity by disabling friction and drag and setting the velocity of the rigidbody directly to a value. you should not aply forces and you should just use the velocity property and set it to the value that you want. generally it's not a good idea to make your character move in this way. you should use ragdolls when you need physics and use character controllers when you don't need it. simulating human movements with physics is not an easy task and it will become very heavy. there is a great video about physics in unite 08. one of the problem that is described there is "character simulation using ragdolls or rigidbodies".

hope this helps