Many SOA proponents would argue that service oriented architecture and cloud are two separate things: SOA is a methodology or philosophy for abstracting and leveraging applications across enterprises, while cloud is a technology platform and delivery mechanism.
Nevertheless, the value proposition of cloud -- providing access to resources via standardized, reusable services -- are a natural extension of what has been achieved with SOA.
So, is SOA now cloud? E.G. Nadhan provides 5 "tell-tale" signs of the transition that has been unfolding:
Rise of as-a-service model. "Infrastructure components, provisioned as services, had to be taken into account as part of the overall SOA strategy. Given the vital role of IaaS within the cloud, a holistic SOA enterprise-wide SOA strategy is essential for successful cloud deployment."
Greater location transparency. "Service orientation introduced location transparency so that the specifics of the physical location where the services were executed did not matter as much. Extending this paradigm, cloud leverages the available resources as and when needed for execution of the services provided."
More virtualization. "Virtualization was extended to infrastructure components facilitating their rapid provisioning as long as it met the experience expectations of the consumers."
Less hardware. Cloud extends the notion of virtualization "into a pool of hardware resources being shared across multiple applications."
Chargeback. Who pays for the creation and upkeep of services? Metering and chargeback mechanisms were developed for SOA services, and now are being applied to cloud.