  The nature of dark energy can be probed not only through its equation of
state, but also through its microphysics, characterized by the sound speed of
perturbations to the dark energy density and pressure. As the sound speed drops
below the speed of light, dark energy inhomogeneities increase, affecting both
CMB and matter power spectra. We show that current data can put no significant
constraints on the value of the sound speed when dark energy is purely a recent
phenomenon, but can begin to show more interesting results for early dark
energy models. For example, the best fit model for current data has a slight
preference for dynamics (w(a)\ne-1), degrees of freedom distinct from
quintessence (c_s\ne1), and early presence of dark energy (Omega_
de(a<<1)\ne0). Future data may open a new window on dark energy by measuring
its spatial as well as time variation.
