Consumer grade GPS chips are designed to quit working when they sense they are traveling above a certain speed and/or altitude. This is for security reasons (think missiles). I used to work at a GPS company. We had an entire Military division that had products controlled by the Dept. of State and Dept. of Commerce for this very reason.
This definitely isn't true for the 2 phones and 2006-era Garmin Nuvi -- I have been able to get a lock on planes, and see the location, altitude, speed, etc using a GPS diagnostic app.
The issue is that the plane acts as a great shield, so you won't generally be able to lock from an aisle or middle seat; it usually requires holding the device right up to the window. And, of course, if you're depending on online maps, you won't have cellular data so may not see the actual map unless cached.