About 18 months ago, I wrote a paper for MaXware about Identity Management in a Service Oriented Architecture (SOA) and described the scenario of initiating provisioning events from enterprise applications via SPML to the provisioning system (now called the Provisioning Service Provider in an SPML scenario).
Martin Raepple of SAP just published an article titled No Limits for Identities. In it, he discusses the process and business value of leveraging SPML for provisioning. He also discusses the role of the Provisioning Service Provider (PSP).
It seems that SAP has done a good job of quickly leveraging one of MaXware's core strengths to enable the NetWeaver platform to act as an open and available PSP for the enterprise. Many of the other major provisioning platforms also support SPML, but I haven't heard of many customers leveraging a service-based provisioning model. I still expect this type of architecture to become more commonly used. Have you seen it in action?