  By comparing with the most recent experimental results, we point out the
model dependence of the present bounds on the anisotropy of the speed of light.
In fact, by replacing the CMB with a class of preferred frames that can better
account for the experimental data, one obtains values of the RMS anisotropy
parameter (1/2 -beta + delta) that are one order of magnitude larger than the
presently quoted ones. The resulting non-zero anisotropy can be understood
starting from the observation that the speed of light in the Earth's
gravitational field is not the basic parameter c=1 entering Lorentz
transformations. In this sense, light can propagate isotropically only in one
`preferred' frame.
