The correct answer is
'the data in each voxel are re-written with an interpolated value that incorporates the values obtained for that voxel in the previous and subsequent aquisitions in some sort of trend analysis'. 3dTshift does the interpolation with FFTs.
I have considered modifying 3dDeconvolve to allow for slice-wise regressors (which is what your alternative would be), but that has yet to be implemented.
bob cox