Stonemonkey,
I dont know about voxels but what you could do is something like:
assuming you have u,v texture coords
normalise (eyevec) <- v important to take care of z component of eye
height = heightTexture(u,v)
u= h * u * fudgefactor
v= h * v * fudgefactor
Fudgefactor is some arbitrarily small number that looks good. That would be parallax mapping. I dont know aboput relief mapping equations.
Chris