I also have a light, cheap, cable lock I can stick in a jersey pocket. I rarely use it, because mostly I would be stopping at a place where I could watch my bike, or take it inside. When I've gone on group rides where we stop at a restaurant, I usually end up locking my bike with someone else's, who has a good lock.
I have to be with DH, when he uses his Carradice bag, to have the super good lock with me. I just had to buy a new seat bag and now i don' even have room for arm warmers or a vest. There's absolutely no seatpost there.