I think it's a secure link since you have to have the WiFi set up for the Phantom and do the "Connect" before you can fly. It's not the same as FREE WiFi at McDonalds....! If anyone with a SmartPhone could hack into the controls, that could make for some interesting conflicts...!
Keep in mind though that the actual flight controls of the Phantom itself are all coming from the 2 stick controller. The WiFi link is only for the FPV...(First Person Video) and the views of the telemetry data...height, speed, altitude, GPS status and that 'birds-eye' view on your iDevice..!