BA lets you book one-way awards online for exactly half the round trip level. I booked 3 BHD-MAN tickets that way this summer.
AC, on the other hand, charges more than half the points and on top of that charges you the telephone booking fee. At the big YYZ do in March 2005 the question I asked at the Q+A session was about the ability to do 1-way award bookings. The answer was that it wasn't a simple thing for the IT guys to do. Obviously not.........