Robot platforms have been proposed to address challenges of wireless sensor network deployments in dynamic environments. Enabling a fraction of nodes to be mobile can lead to improvements in sensing coverage, network lifetime, and network connectivity. We present the design of ArmBot, a Cortex-M3 based robot platform, that natively supports TinyOS applications and system services. ArmBot is cost-efficient as it is built from off-the-shelf hardware components and versatile due to its power-efficient ARM processor core. We evaluate the platform in the context of core sensor network services, such as time-synchronization, and analyze its energy efficiency under varying mobility and computation loads.