Mobility footprint refers to the size, weight, and energy demand of the hardware that must be carried by a mobile user to be effective at any time and place. The ideal of a zero mobility footprint is achievable by encapsulating personal computing state in a virtual machine (VM) and delivering it over the Internet to a locally-obtained computer close to the user. In locations with poor Internet connectivity, the demands placed on WAN bandwidth can result in unacceptable user experience. We show how this challenge can be overcome by using nascent smart phone technology as a trusted personal assistant called Horatio that serves as a self-cleaning portable cache for VM state. Since most users already carry cell phones for voice calls and texting, Horatio does not increase the size or weight aspects of a user's mobility footprint -- there is only a small increase in the energy aspect. We have built an experimental prototype of Horatio, and measurements confirm its ability to improve u...