  A number of general issues relating to superluminal photon propagation in
gravitational fields are explored. The possibility of superluminal, yet causal,
photon propagation arises because of Equivalence Principle violating
interactions induced by vacuum polarisation in QED in curved spacetime. Two
general theorems are presented: first, a polarisation sum rule which relates
the polarisation averaged velocity shift to the matter energy-momentum tensor
and second, a `horizon theorem' which ensures that the geometric event horizon
for black hole spacetimes remains a true horizon for real photon propagation in
QED. A comparision is made with the equivalent results for electromagnetic
birefringence and possible connections between superluminal photon propagation,
causality and the conformal anomaly are exposed.
