One of the terms associated with cloud computing at the moment is Cloudbursting.
Using current cloud computing models for handling all of your applications and traffic can be costly, and using it for handling the peaks in your usage patterns can also be fraught with problems because of the time required to get the system set up and seeded with the information. Ramping up in the cloud is difficult since it may take minutes to fire up more hosts but your peak requirement may last less time than that.