I tried my GPS from a plane while in flight ^^

11000 meters and 810 Km/h
or about
36000 feet and 500 mph

Actually, It's legal, since the GPS system only receive data and do not transmit at all...because any transmitting device is prohibited at all time on a airplane...while the use of non-transmitting device is allow above 3000 meters (10000 feet) you should avoid going through an argument with the crew about the technicality of the GPS system...and proceed very carefully not to be noticed by the crew.

for additional information -> http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/b5c85ae8e38df676862571da00529da4/$FILE/AC%2091.21-1B.pdf