The airline I work for has recently introduced laptops in the flightdeck. In order to gain CAA approval for this the WiFi transmitter has to be disabled during the flight. Our systems department have found a way to do this and so whilst no one really knows what the effects might be, the answer is as far as the CAA are concerned - NO, the WiFi transmitter must be disabled before flight. If you cannot do this then the laptop must not be used.
The general rule is that anything which has the capability to transmit on a radio frequency is banned - this is why Furbees were not allowed a few years back.
Lazlo