Architectural design has an important effect on usability, most notably on temporal properties. This paper investigates software architecture options for mobile user-interfaces, in particular those for collaborative systems. One of the new features of mobile systems as compared with fixed networks is the connection point to the physical network, the point of presence (PoP), which forms an additional location for code and data. This allows architectures that bring computation closer to the users hence reducing feedback and feedthrough delays. A consequence of using PoPs is that code and data have to be mobile within the network leading to potential security problems.