Benualdo posted in the Light Pre-Pass Thread a cool trick on how to detect edges to run a per-sample shader for MSAA (just in case centroid sampling doesn't work for you). Here it is:
another stupid trick for edge detection pass on platforms that support sampling the MSAA surface with linear sampling: sample the normal buffer twice, once with POINT sampling and once with LINEAR sampling. Use clip(-abs(L-P)+eps). The linear sampled value should be used to compute the lighting of "non-MSAA" texels in the same shader to avoid an extra pass.
eps is a small threshold value to bias the texkill test so that when the multisampled normals are only a little different then we could use the averaged value to perform the lighting at non-MSAA resolution during the first pass as an optimization.