If GLAST can see up to 300 GeV, AND the observations from MAGIC and Markarian 501 were correct, then at the highest energies, GLAST should see a 10-30 second delay (at 450 million light years).

IF the short burst GRBs give off gammas at that energy (not an easy assumption) this effect should be pretty obvious, and clearly defined.

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Forming opinions as we speak