the most recent LV paper I know of was just revised and reposted 18 December 2003 http://arxiv.org./abs/astro-ph/0309681 F. Stecker is one of the co-authors. "New limits on Planck scale Lorentz violation in QED" T. Jacobson, S. Liberati, D. Mattingly, F.W. Stecker Nereid recently recommended an earlier paper by Stecker (solo) http://arxiv.org./abs/astro-ph/0308214 "Constraints on Lorentz Invariance Violating Quantum Gravity and Large Extra Dimensions Models using High Energy Gamma Ray Observations" Stecker is at NASA Goddard, and the other three authors are at nearby U. Maryland. All have written several papers on tests of LV, Jacobson especially---it's a long-standing interest of his I believe. Stecker has a review paper on high-energy cosmic rays which touches on tests of Lorentz violation among other topics "Cosmic Physics: The High Energy Frontier" http://arxiv.org./abs/astro-ph/0309027 However the most recent thing we have is, I think, the one I just mentioned by Jacobson, Mattingly, Liberati, Stecker. It was revised and reposted yesterday and is worth checking out---here's an exerpt: "Constraints on possible Lorentz symmetry violation (LV) of order E/MPlanck for electrons and photons in the framework of effective field theory (EFT) are discussed. Using (i) the report of polarized MeV emission from GRB021206 and (ii) the absence of vacuum ¡Cerenkov radiation from synchrotron electrons in the Crab nebula, we improve previous bounds by 10−10 and 10−2 respectively.... ...While no current suggestion of LV is firm enough to be considered a prediction, there is nevertheless great interest in the possibility of LV induced by Planck scale physics since it offers the hope of an observational window into quantum gravity... ...We use the reported observation  of polarized gamma rays from the gamma ray burst GRB021206 to improve the birefringence constraint by ten orders of magnitude.1 By consideration of the vacuum ¡Cerenkov process for the electrons producing the highest frequency synchrotron radiation from the Crab nebula we improve on the old birefringence constraint by two orders of magnitude... ...We adopt the framework of effective field theory as developed e.g. in [1, 2, 3], focusing on the electron-photon sector since this involves no other particles and there are many observations allowing a number of independent constraints to be combined. We assume rotational symmetry is preserved in a preferred frame, which is taken to coincide with that of the cosmic microwave background..." I've added the bolding for emphasis. Another useful reference is a Jacobson Liberati Mattingly paper revised and reposted 29 August 2003 "A strong astrophysical constraint on the violation of special relativity by quantum gravity" http://arxiv.org./abs/astro-ph/0212190 Qualitatively it seems that Stecker and the later papers are doing very much the same thing as this paper only making the constraints progressively tighter. So looking at this paper (which spells things out in considerable detail) can help understand this line of research. Here is an exerpt: "...Here we determine a very strong constraint on a type of Lorentz violation that produces a maximum electron speed less than the speed of light. We use the observation of 100 MeV synchrotron radiation from the Crab nebula..."