Just how much of the system is getting shut off in these new C-states? A lot of the best power supplies have a single 12V rail, and the 3.3V and 5V rails are provided by stepping down the voltage from the 12V rail. It seems like any PSU with that kind of design wouldn't be affected unless the total system power consumption drops below 6W, which would require shutting off at least the expansion cards, disk drives, any power-hungry USB devices, and potentially the case fans. Even 4 DIMMs plus the keyboard and mouse necessary to wake up the system might be sufficient to keep the power draw high enough.