  We show that measurements of stellar proper motions in dwarf spheroidal
galaxies provide a powerful probe of the nature of dark matter. Allowing for
general dark matter density profiles and stellar velocity anisotropy profiles,
we show that the log-slope of the dark matter profile at about twice the
stellar core (King) radius can be measured to within \pm 0.2 when the proper
motions of 200 stars are added to standard line-of-sight velocity dispersion
data. This measurement of the log-slope provides a test of Cold and Warm Dark
Matter theories at a sensitivity not possible with line-of-sight velocity
dispersion measurements alone. The upcoming SIM PlanetQuest will have the
sensitivity to obtain the required number of proper motions in Milky Way dwarf
spheroidal galaxies.
