Captain Stable,
Come on, You could say that about any device.
Laptops could hold aircraft layout details to help the hijacker plan how the operation will work.
The same can be said for palm PC's, Some have infra-red which hijackers could use to send messages to each other.
Airband scanners could be used to monitor the progress of the flight to enable the hijacker to make their move.
At the end of the day if a highjacker wants to bring a flight down he or she (Yes, there have been female hijackers) will do it.
A hijacker strapped with explosives will simply blown themselves up and not worry too much where the plane is, They will not be fiddling around with GPS receivers.