Mordred - light always travels at c. In a Bose-Einstein condensate (not Bohr's!) or another thick substance, the slowing of light is just an apparent effect as photons are constantly absorbed and re-emitted. Bending of light in such a medium would be the same thing - an illusion.
Jerromy - You can interpret the curved paths objects take in gravitational fields however you'd like. If you want to see them as objects following curved paths in flat spacetime, feel free, but when you write down the mathematical expression for those paths, you'll be shocked by their uncanny resemblance to geodesic paths in curved spacetime!
As it is, there's no a priori reason for spacetime to be flat, except that we happen to like it. We know of no physical process which should restrict the curvature to be flat, so it seems obvious we should consider general spacetime curvatures and see what happens.
A final thought to consider on this: classically, we expect light to follow least time paths. This is how, for example, Snell's law is derived; light which refracts according to that rule will reach its destination faster than light following any other path. Maxwell's laws, which give rise to electromagnetic radiation like light, predict the same. In the absence of a good reason for light to suddenly behave differently when gravity is involved, doesn't positing that run against the spirit of Occam's Razor?
Light does change wavelength as it moves towards/away from a gravitational source, due to conservation of energy and the constancy of the speed of light.