Re: I think _I_ can explain it (and it's not that hard)
"(and if photons were truly 'massless', gravity would not affect them)"
Gravity, caused by the presence of matter, makes space itself curved and so a photon follows the curvature of space as it travels. The question is, has anyone performed experiments on trapped photons to determine if they are affected by gravity (attracted to matter) when they are fairly stationary?
If you're equating the energy of a photon to an 'effective mass' via an e=mc^2 equivalence, then you'd have a situation in which X-ray photons have a larger effective mass than light photons and hence should be deflected more by gravity.
I'll wait for the next commentard to add to this thread by explaining why I'm wrong.