an easy test i did to see the vibration dampening effects of a carbon vs aluminum fork was held the fork at the end of the steerer tube and hit one leg with the base of my hand (like a tuning fork) and counted how long i felt the vibration. The aluminum fork vibrated about 25-30 seconds, the carbon fork vibrated about 1-2 seconds. I know this doesnt exactly happen in the real world (a fork taking a blow to the side of one of the legs without a wheel mounted or stem etc) but i think its a good example of what happens and what the properties of each material are.
As a side note, riding on a carbon fork felt so much more cushy to me, as if my tire pressure had dropped 20 psi or so.
ive only tried riding on a crappy steel fork, and was unimpressed, but that could be because it cost me only $20 new, so that doesnt count.
jeremy