There is a very good review article on the consequences of the mass of the photon, and a list of experiments that tried to detect such consequences. The paper is L.C. Tu et al., Rep. Prog. Phys. v.68. p.77 (2004). Most people forget that the possible mass of a photon effects not only SR and GR, but also the classical maxwellian description of light. Such postulate will produce a number of consequences, such as the variation in the speed of light for different frequencies, etc. The list of experiments that try to test for these consequences have put an increasingly stringent upper limit on any possible mass of a photon. I should also point out, for people who are not in this field, that this is another example where, if one makes a claim or postulate (such as "a photon has mass"), then there has to be a series of measurable consquences. This is what distinguish physics (and science in general) from pseudoscience and quackeries. Also note that while we accept photons to have no rest mass, it still doesnt't stop us from continually testing this postulate. The often-made accusation that we simply and blindly follow what we have been taught is clearly false here. Zz. P.S. If you've read my Journal entries, then you would have had this info already. If you haven't, take note that the article in question here is published by the Institute of Physics (IoP). The online edition of ALL of IoP papers can be accessed for the first 30 days that they appear electronically FOR FREE! (You may need to register first, I think). So even if you have no subscription, you can still get this article. Just go to the IoP journal website at http://www.iop.org/EJ/S/3/1350/ and look for the relevant journal with this article. This particular article just appeared either today or yesterday. So you have barely a month left to get free access to it.