My thanks to jtbell for this timely rebuke. What you get is the amazing result that v
differs from c by only 1 part in 10-34
. Even over a distance of 700km this would slow a photon down by totally negliible amount. Not nearly enough to explain the OPERA results.
gives some interesting information about experiments to measure the mass of a photon. Apparently the most sensitive experiments involve measuring the deviations from Coulombs inverse square law. It also transpires that if photons had significant mass, charge would not be conserved either. (Thanks again to Simon for pointing me in the right direction)
On balance, I think we had better stick with a zero rest mass for now and look for another explanation of the OPERA results.