The iPhone does not use 3G for GPS. It has a built in GPS receiver. What it does is use A-GPS to aid the standard GPS chip. A-GPS allows it to fix its position from cold in a matter of seconds by using triangulation from cell sites to establish the correct almanac to use.
If the phone is out of range of a cell site such as in the air then the chip operates as a conventional chip and has to search the sky for the satellites, secure the almanac download and the fix position. It does this the same as you cold starting a garmin out of the box and can take several minutes in order to gain a lock this way. If the phone goes into screensaver mode and you are not using an application that prevents it then the GPS is shut down again to save power. You then end up in a cycle of cold starts that never allow a lock to be made.
The iPad comes in two flavours. The Wifi and Wifi plus 3G. Only the 3G version has a GPS chip installed. So if you are trying to use a non 3G version airborne you will never get a fix as there is no GPS in it!
You can buy an external GPS receiver that will mate to the phone and the iPad that works really well, stick to the windscreen and off you go. There is even an XM enabled version now.