  We present the results from a series of lattice simulations of the charmonium
system using a highly-improved NRQCD action, both in the quenched
approximation, and with n_f = 2 light dynamical quarks. The spectra show some
evidence for quenching effects of roughly 10% in the S- and P-hyperfine spin
splittings---probably too small to account for the severe underestimates in
these quantities seen in previous quenched charmonium simulations. We also find
estimates for the magnitude of other systematic effects---in particular, the
choice of the tadpole factor can alter spin splittings at the 10-20% level, and
radiative corrections may be as large as 40% for charmonium. We conclude that
quenching is just one of a collection of important effects that require
attention in precision heavy-quark simulations.
