The pixel size is still around the same as smaller monitors. You would have to sit far away from it to not notice the jagged edges. Maybe if you had that IBM T221 with its 3840x2400 resolution on a 22" screen, AA may not be quite as important, but this new Dell has a higher pixel size than my monitor at its maximum resolution, at which the jagged edges are still quite apparent with no AA at all.
That being said, I doubt there is much of a difference beyond 4x combined AA, since at higher resolutions small levels of AA tend to remove the jagged edges completely, which is different from even heavy AA at low resolutions.