Sodium cobaltate has excellent thermoelectric properties, including low thermal conductivity. We have measured its phonon dispersion using inelastic x-ray scattering, and we find that it has well defined modes unlike a phonon glass. First-principles density-functional calculations are in excellent agreement with the data, and we identify a rattling mode. We now propose to use the better energy resolution of inelastic neutron scattering on IN8 to measure the energy line widths of selected modes as a function of temperature. This will allow us to determine the phonon lifetimes and estimate the lattice contribution to the thermal conductivity.