There is no reason why the "speed of light" cannot be exceeded if the "vacuum" is altered (analogous to space being "warped"), and no, it would not make the light go back in time! That the vacuum can be altered ("rarified" for lack of a better term), at least to a very small extent, is already demonstrated by the Casmir Effect. Recent experiments have already suggested that neutrinos can exceed the speed of light, throwing "conventional" notions into question.
There may also be "non-linear" effects. At high enough energies, a particle could potentially interract with the vacuum, changing the "speed of light" it experiences. Since the vacuum energy is extremely dense, we are unlikely to see such effects demonstrated in particle accelerators any time soon.