We speak of the cloud, but we have no cloud. We have clouds: private clouds, a cloud at Amazon, a cloud at Rackspace; more and more clouds, distinct and separate. Enterprises looking to cloud computing may well decide on an all-of-the-above strategy, picking and choosing clouds based on cost and quality. Or they may go hybrid, keeping their data center but extending it into the cloud when and where that makes sense.
Making this choice possible requires both connecting and isolating clouds: connecting so that computers in the enterprise and computers across multiple cloud service providers can communicate; isolating so that this communication stays within the bounds of the enterprise.