Chuqui has a good take on the possibility of a non-user changeable battery over at Chuqui 3.0.
I only have a couple things to add: 1) This approach might lead to Apple actually licensing the MagSafe to some companies to make useful things such as car chargers; 2) I think a second AC adapter is much more useful than a second battery. Leave it plugged in where you usually compute at home and keep one in the bag for remote use.