On the 4th of December 2009 there was a meeting of the MR physics & analysis for cognitive neuroscientists group on DTI, which included a discussion about the new CBU standard protocol for acquisition of diffusion weighted images.
Here you can find some generic guidelines for chosing the voxel size/shape for you diffusion MRI experiment. Diffuson_voxel_size.doc
The new standard CBU DTI sequence acquires isotopic voxels with dimensions 2x2x2mm3. The gaps between slices have also been removed, and instead we use interleaved slice acquisition to avoid crosstalk artefacts (more information here Desc_vs_Int.doc ).