This isn't super-technical, but Mythbusters visited this question twice. In their second attempt, they found that the break-even point was 50 mph. (http://en.wikipedia.org/wiki/MythBusters_(2005_season)#Episode_38_.E2.80.93_.22MythBusters_Revisited.22)

Going less than 50mph it is more efficient to leave the windows down, but going greater than 50mph it is more efficient to use the A/C.