Wireless portable devices and ubiquitous service access points are enabling pervasive scenarios where a plethora of different client terminals expect to access services while roaming from a network locality to another one and between different local service providers. Within this usage perspective, there is the need for extending accounting functions to include also the possibility to charge clients for application-level resource consumption, i.e., application domain accounting. The paper calims that application domain accounting of roaming services requires extending the fixed Internet infrastructure, where and when needed at service provisioning time, with mobility-enabled monitoring, processing, pricing and charging functions. We propose an accounting middleware solution capable of evolving dynamically in dependence of the exhibited mobility patterns of Wi-Fi client devices. The presented solution is based on the Mobile Agent (MA) implementation technology, on portable mechanisms for quality monitoring, and on portable mechanisms for achieving the visibility of IEEE 802.11b client roaming at the middleware level to automatically trigger the migration of the needed middleware components.