In the cloud lock-in is often a greater concern than lock-out*, but both should be top of mind for organizations looking to deploy cloud seriously, or better said, for organizations that want to deploy cloud for serious business.
Lock-in is the phenomenon that it is often difficult (lengthy and expensive) for user organizations to switch suppliers, often because after some time the software becomes so closely linked with the way the organization works that saying goodbye to the vendor becomes virtualy impossible. It is no coincidence that there are only two industries (IT vendors and drug dealers) that routinely refer to their customers as «users».