(See May 31st 2007 press release titled Garmin Adds High Sensitivity GPS Receiver to eTrex® Series here.) I can confirm it cannot pick up satellites inside the aircraft (A-320) unless it's placed right next to the window, as shown above. The window shade and the rubberized case cooperate nicely. Then it happily tracks the flight. I wonder how much better the model with the newer chipset performs.
[I turned on the GPS somewhere before the plane got near Chicago and turned it off shortly before landing at Phoenix.]