This question came up a while ago..
...My company most definetaly prohibit the use of hand held GPS units in flight !
This arose as the result of a report to the CAA concerning an incident that resulted in "in-flight interference caused by a handheld GPS unit", from another UK based airline.
These toys are receivers and there use on board an aircraft is as irresponsible as is using a mobile telephone or a radio receiver.
Wait until you arrive at your destination and play with them all to your hearts content.
As a matter of policy it is ultimately a decision for the airline, the regulatory authority and finally the Captain.