So, as far as I see, we are at a stand-off, where we have arguments in favor of a late settling of the ACC (mostly sedimentary and tectonic arguments) and at the same time arguments in favor of an early settling (in particular paleontological and geochemical arguments) and no real reason to discard any.
References:
Barker & Thomas, 2004. Origin, signature and palaeoclimatic influence of the Antarctic Circumpolar Current. Earth-Science Reviews, 66: 143-162.
De Conto, R.M., Pollard, D., 2003. Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric $\mathrm{CO_2}$. Nature 421, 245 – 249.
Kennett, J.P., 1977. Cenozoic evolution of Antarctic glaciation, the circum-Antarctic Ocean, and their impact on global paleoclimate. Journal of Geophysical Research, 82: 3843–3860.
Lazarus, D. B. & J.-P. Caulet, 1993. Cenozoic Southern Ocean reconstructions from sedimentologic, radiolarian and other microfossil data. In J. P. Kennett and D. A. Warnke (Eds), The Antarctic Paleoenvironment: a perspective on global change. Antarctic Research Series, 60: 145-174.
Lazarus, D. B. et al., 2008. Patterns of opal and radiolarian change in the Antarctic mid-Paleogene: clues to the origin of the Southern Ocean. Micropaleontology, 54(1): 41-48.
Lyle et al., 2007. Late Oligocene initiation of the Antarctic Circumpolar Current: Evidence from the South Pacific. Geology, 35: 691-694.
Scher, H. D. & Martin, E. E., 2006. Timing and Climatic Consequences of the Opening of Drake Passage. Science, 312: 428-430.