Wireless data services, other than those for electronic mail or paging, have thus far been more promising than successful. We believe that future mobile information systems must be built upon heterogeneous wireless overlay networks, extending traditional wired and internetworked processing `islands' to hosts on the move over coverage areas ranging from in-room, in- building, campus, metropolitan, and wide-areas. Unfortunately, network planners continue to think in terms of homogeneous wireless communications systems and technologies. In this paper, we describe a new wireless data networking architecture that integrates diverse wireless technologies into a seamless internetwork. In addition, we describe the applications support services needed to make it possible for applications to continue to operate as mobile hosts roam across such networks. The architecture described herein is being implemented in a testbed at the University of California, Berkeley under joint government/industry sponsorship.