  We study the flow of concentrated hard-sphere colloidal suspensions along
smooth, non-stick walls using cone-plate rheometry and simultaneous confocal
microscopy. In the glass regime, the global flow shows a transition from
Herschel-Bulkley behavior at large shear rate to a characteristic Bingham slip
response at small rates, absent for ergodic colloidal fluids. Imaging reveals
both the `solid' microstructure during full slip and the local nature of the
`slip to shear' transition. Both the local and global flow are described by a
phenomenological model, and the associated Bingham slip parameters exhibit
characteristic scaling with size and concentration of the hard spheres.
