App = Application.
Sure, it'll work but don't stake your life on it. The GPS lock acquisition speed on the 3G/3GS can be fairly slow and GPS doesn't work with the phone in Airplane Mode. It's aGPS (assisted) aswell, meaning it uses 3G triangulation to speed up the initial lock speed. You'd have to mount it somewhere far enough away from radio/headset to prevent it pipping in your ear.
For anything approaching acceptable lock speed for airborne use, you'd really want to use an iPhone 4 or iPad (the latter is quite good with a Brodit mount kit, particularly with an anti-glare screen cover). However, also worth noting that all it does is show you where you are on a map, it doesn't know where any of the airfields are etc...