I came across this awesome finish on Thefauxshop.com. I am loving it!
In order to achieve this plaster stamped finish, you would first apply your plaster to the wall and then while it is still wet lightly press the stamp into the plaster and gently pull it away, leaving the stamp imprint. Once the finish is completely dry, you can go over it with sandpaper (if using a tinted plaster) to lighten the raised areas. Or you can use a glaze wash, if using a white paint-able plaster.
Please feel free to contact me if you would like more details on how to achieve this finish.