Are GRB shocks mediated by the Weibel instability?
  It is estimated that the Weibel instability is not generally an effective mechanism for generating ultrarelativistic astrophysical shocks. Even if the upstream magnetic field is as low as in the interstellar medium, the shock is mediated not by the Weibel instability but by the Larmor rotation of protons in the background magnetic field. Future simulations should be able to verify or falsify our conclusion. 
Yuri Lyubarsky, David Eichler