Not sure about that (planes being radio-sealed). Planes are getting more and more composite materials to save weight. One of the key features of the 787 is the composite fuselage that I am not sure will effectively block GPS signals. Also, many cargo planes are remissioned or dual-use passenger planes that still have windows:

http://picasaweb.google.com/lh/photo/XMBDcG2dmr-Z1QoGBdXu2g

If all else fails, some phones are coming with gyroscopes...