I got hammered with a data overage from ATT and was thinking maybe it's enabled but there is no longer a toggle available. I'm on a lowly 200MB plan and there was a 193MB transfer all at once I can't really account for, especially since I have downloading over 3G turned off under the iTunes and App Store section in Settings.
Maybe someone could disconnect their WAN connection from their wireless router while on Wi-fi and see if their download automatically continues by using cellular?