That's something I've wondered for a while now is that you are strictly not allowed to turn on your mobile phone in flight, but your laptop with its wireless network card can transmit away all it likes looking for a network when you boot it up to use it in flight.
When WiFi first came out, I asked the head of QF Flight Ops Technical whether it posed similar problems to mobile phones. His response was that WiFi signals are much lower in power than mobile phones and have been assessed as having no risks to aircraft. When you think about how short WiFi ranges are in comparison to mobile phone range then it makes sense.