  We revisit the constraints that the non-observation of ultra-high-energy
photons due to the GZK cutoff can impose on models of Lorentz violation in
photon propagation, following recent work by Maccione, Liberati and Sigl
[arXiv:1003.5468] that carries further an earlier analysis by the present
authors (Phys. Rev. D 63,12402 (2001), [hep-th/0012216]). We argue that the GZK
cutoff constraint is naturally evaded in the D-brane model of space-time foam
presented recently by the present authors (Phys. Lett. B 665, 412 (2008)
[arXiv:0804.3566]), in which Lorentz-violating effects on photon propagation
are independent of possible effects during interactions. We also note a novel
absorption mechanism that could provide a GZK-like cutoff for photons in
low-scale string models.
