In the U.S., this almost certainly depends on jurisdiction. My understanding is that many places have laws that provide only basic default speed limits and require drivers drive at prudent and safe speeds. Violating posted limits constitutes prima facie evidence that your speed is not reasonable or safe, but the offense isn't that you were doing X in a Y. Given that, the exact placement of the sign might not even be meaningful. The real question is where the conditions which warrant a reduction in speed begin, and whether your speed was prudent and safe at the time the alleged violation occurs.
Under this theory, if the speed limit were changing for no good reason and you could prove it in court, you might beat the ticket. I wouldn't bet a large amount of money on such a venture being successful.
Perhaps an interesting legal question is at what point the posted limit creates the conditions for recorded speed becoming prima facie evidence of imprudent or unsafe driving. Again, my gut says that the zone begins as soon as:
a reasonable man driving at a prudent and safe speed in the same situation would become aware of the posted limit; and
the conditions which justify the reduction in speed (a curve, intersection, construction, etc.) occur