In recent years, the development and deployment of new wired and wireless access network technologies have made the ubiquitous Internet a reality. Users can access anywhere and anytime to the broad set of value-added Internet services, which are delivered by means of the IP protocol. In this context, 3GPP is currently developing the IP Multimedia Subsystem (IMS), as a key element that allows to evolve from the ubiquitous access to the Internet services towards a next generation network model, by providing a set of essential facilities such as session control, QoS, charging and service integration. Nevertheless, several open issues still need consideration before the future Internet becomes real, such as supporting user mobility in IP networks. Although mobility support in the Internet is receiving much attention, IMS networks present inherent particularities that require further analysis. The solutions proposed so far for IMS do not support mobility transparently to the end-user applications, or address the problem by introducing complex changes to the IMS infrastructure. This paper presents TRIM, an architecture for transparent IMS-based mobility. TRIM supports mobility in IMS networks transparently to the end-user applications, which are unaware of the handover management procedures executed between the mobile node and the network. We have performed several experiments with a TRIM prototype, using a real IMS testbed with 3G and WLAN access networks, validating the proposal for UDP and TCP based applications.