My iphone has picked up gps signals on short flights on several offasions, I have an almost flawless track on a flight from LGA to DCA, and on over the Mediterranean from Frankfurt to Lagos.

The LGA to DCA one had a max speed of 461mph (average 322) and altitude of 23,400 foot, with maximum negative gradient of 67.7%.

Maximum speed I've recorded overall (I've had the app open for 24,190 miles) is 673.3mph, and max altitude of 35,081 ft