The amount and spatial distribution of dark matter in elliptical galaxies are poorly known, despite extensive observations with multi-object spectrographs on 4-10m-class telescopes. We examine the prospects for measuring the structure of dark matter halos in elliptical galaxies — and the orbital properties of their planetary nebula and globular cluster systems — from radial velocity measurements made with WFOS, a wide-field optical spectrograph operating on the Thirty Meter Telescope (TMT).