^True. However, there is something to be said for being able to attract Nobel-calibre researchers. Also, ARWU assigns less weight to prizes that were awarded several years in the past.
I hope to see Duke rise up the rankings as its faculty gets the international recognition it so richly deserves. Two Nobel laureates (three if you count former post-doc Kobilka) in the last four years is nothing to scoff at. The investments that schools like Duke made in the early 80s are reaping rewards. The prizes have been dominated by a handful of institutions for too long (and the reason for this was probably more political than scientific). Duke, Michigan and Stanford are coming to the party in a big way now that the Ivy hegemony is diminishing.