Today, we are seeing an increasing number of software applications that users want to use anywhere, anytime. Such mobile applications often deliver their user interfaces (UIs) to client devices over the World Wide Web. However, Web-based UIs cannot provide the same level of usability as window-based UIs on mobile devices with their small screens and occasional network dropouts. To address this challenge, we present a UI framework that combines the usability of a full-featured UI with the flexibility of a thin presentation logic: We exchange interface specifications and events between the application logic on the server and a generic UI rendering engine on the client device using an instant messaging infrastructure. The paper gives an overview of the framework architecture and the features of the communication protocol, and discusses performance measurements obtained on a public network.