This question pops up regularly. Shawn blogged about the differences between collaboration and cooperation here and here. The Economist Intelligence Unit provide a paper here describing the difference between collaboration, coordination and cooperation, using trust as one of the key differentiators.
Yesterday, I came across this excellent short video from Phil Culhane of the Collaboration Lab in Ottawa. It describes the difference between communication, consultation and collaboration. In this construct, the main differentiator is accountability. In essence, collaboration will only occur where both (all) parties are accountable for what happens.