Experimentation with large mobile networks is notoriously tedious and expensive. We present the architecture and work-in-progress implementation of the m-ORBIT testbed, a mobility emulator using spatial switching, which facilitates mobile system experiments with 802.11a/b/g wireless network interfaces. The emulator does not require any physically moving parts---it emulates mobility by switching over an array of 128 spatially distributed radios. Instead of using hardware antenna switches, we implement spatial switching in software over Gigabit Ethernet links to the radio nodes. Preliminary results support the scaling of this approach to a large number of radios at relatively low cost. Packet error rate measurements also indicate that an experimenter can create multi-hop topologies by injecting additive white Gaussian noise into the environment. We demonstrate through an Ad hoc On Demand Distance Vector routing case study how this emulator enables mobile systems experiments and plan to make the emulator available for remote access by the research community.