1. A system for dynamic resource allocation in a transactional middleware machine environment, comprising: one or more microprocessors; a deployment center running on the one or more microprocessors in a transactional domain that includes a plurality of virtual machines and a plurality of physical machines; a plurality of deployment agents, each of which executes on one of the plurality of virtual machines, and is maintained by the deployment center; wherein the deployment center operates to receive resource usage information from the deployment agent on a first virtual machine of the plurality of virtual machines, wherein the first virtual machine is bound to a first physical machine of the plurality of physical machine for executing an application package, and dynamically scale up or down resources being used in the transactional domain based on the resource usage information; wherein the scaling up includes binding a second virtual machine of the plurality of virtual machines to a second physical machine of the plurality of physical machines, and activating the second physical machine and the application package on the second virtual machine; and wherein the scaling down includes unbinding the first virtual machine from the first physical machine and deactivating the first physical machine and the application package on the first virtual machine.