It really looks like the US marine (David Motari) really did throw the puppy off the cliff.
This is a screen shot of a comment by him from his profile on (http://www.bebo.com/)
UPDATE: APRIL 11th 2008
US Marine Expelled for Throwing Puppy Off Cliff
Excerpt from his www.Bebo.com/DavidMotari profile:
Here is the actual video of the puppy being thrown.