Visitors to any of the Olympic events can bring smartphones and other devices, but are not allowed to bring mobile hotspots or 3G hubs, these are prohibited along with many items (you'd usually expect) and are listed here. Though difficult to police this is probably just to avoid WiFi interference as it's really difficult to maintain healthy signals when there are lots of random devices transmitting. There are also possibly security implications as any WiFi traffic going through the official access points can be logged, while random 3G access points would require tapping at the 3G network, which isn't so easy. Policing (if it happens) will probably be reactive when someone complains they cant access WiFi when they should be able to. Then searching out rogue hotspots is relatively easy and they can be quickly located and shut down (and anybody using a rogue 3G hotspot can be ejected and banned from future events).