Considerations around placement of an ESB and integration into existing architectures carry over to SOA Gateways acting in the same capacity. Some enterprises require a hardware gateway form factor when dealing with deployments that touch the DMZ, some deploy all internal applications on virtualization software and others need their internal architecture to be replicable in a public cloud environment. These form factor choices will be of primary importance to your network and operations teams, along with clustering methodologies and integration with existing logging, monitoring and reporting.
Understanding the limitations of SOA Gateways is equally important. They're powerful and flexible, but are by no means a panacea. For example, though they generally provide workflow and logical message processing capabilities, they are not BPEL-enabled business orchestration engines that can manage all of your human interaction and long-running processes.
Though they can process batches of messages, they are neither a Managed File Transfer platform nor an ETL data warehousing tool. And though there is definitely some feature overlap with Web Application Firewalls, SOA Gateways don't generally fulfill all of those requirements; neither do WAFs come anywhere close to parity on non-HTML traffic.
The exact deployment model for an SOA Gateway in an ESB role depends on the feature comparison exercise we just went through, and generally falls into one of three categories.
If these "Lightweight ESBs" meet all of your corporate requirements for application integration and SOA deployment, they can easily stand alone and fulfill the ESB pattern. If, on the other hand, an existing ESB is meeting most of your application integration needs, then an SOA Gateway can be deployed as a complement to provide value as an on- and off-ramp to that ESB. This "ESB Gateway" use case focuses on the gateway's strengths around security, high-performance transformation and edge-based protocol mediation.
The third option is most common in large enterprises that have grown through mergers and acquisitions and have a heterogeneous corporate IT landscape. In these cases, the SOA Gateway can perform all of the ESB functions for those divisions without an existing infrastructure and can act as a bridge between other more entrenched technologies in the rest of the enterprise. It even enables extension of this secure architecture to applications deployed in public or private cloud environments. This "Federated ESB" use case takes advantage of the true agility and flexibility of SOA gateways without requiring a rip-and-replace implementation.