Milt - Sorry chum, A-level physics wins. Yes, the force produced by the air that acts on the aeroplane comes from the difference in momentum between the aeroplane and its surroundings, and yes, the only direct link between the Earth and the aeroplane is gravity, BUT, in just the same way that the Earth has it's own momentum travelling through space, so does the aeroplane. This momentum can most easily be referenced to that of the Earth itself, hence groundspeed is relevant. Of course, strictly speaking you need to consider the momentum of the aeroplane through 3-D space, probably referenced to orthoganal axes, but I fear that would be getting far too complicated.
At least, that's the way I've always understood and been taught it, the various poor buggers who've had to teach me could all have been clowns. Or... physics could work completely differently when you're on the wrong side of the world, surely you guys learn this from having to coordinate with opposite rudder?
Islander - true, instantaneous turns are indeed impossible, was just using a rather contrived example. As I said, the actual change is very progressive throughout the turn.
Edited to add lame Aussie banter