As widespread deployment of the IP Multimedia Subsystem (IMS) begins to solve the fundamental challenge of connectivity across network domains and lays the groundwork for value added services, application and service infrastructure will face new challenges. The convergence of legacy telecommunications networks with all-IP infrastructure offering data, voice and multimedia services requires a flexible and robust, execution environment for network applications. Cost-effective value-added network services require application server platforms provide a high degree of availability, seamless interoperability with legacy and next-generation systems, high performance, and a development environment that drives rapid service creation, testing and deployment. The IMS promise of lowering capital and operational costs for service providers while providing an exhaustively expanded range of value added services can not be realized unless the service and application infrastructure accurately reflect the requirements of a real, operational network. The role of the IMS SIP application server in this resulting hybrid architecture is more central, and critical to service provider success, than ever before.