Advances in wireless solutions and portable devices are enabling new challenging service scenarios where mobile users are willing to access ubiquitous and continuous services. This calls for novel middleware capable of tailoring service contents to client characteristics and of following client movements at provision time. The paper proposes MUM, a dynamic and flexible middleware to support continuous services to mobile users in ubiquitous scenarios. MUM performs service configuration by dynamically distributing middleware components to intermediate nodes along the client-server path and provides service session continuity by automatically migrating the session state in response to user movements during service provisioning. MUM exploits mobile agents to move both middleware components and session state, where and when needed, while it allows service developers to continue using the traditional client/server model for MUM-based application components. In addition, The work presents the implementation of a Video-on-Demand service on top of MUM, with the goal of verifying the feasibility of our approach when applied to the challenging multimedia application area. First experimental results show that, notwithstanding the application-level approach, the MUM configuration/session migration times are compatible even with the strict requirements imposed by multimedia distribution over the best-effort Internet.