AFAIK they will find out at the airport and they won't be able to change the seat. If this was the case then Easy wouldn't succeed in seating as many people together as possible....it just wouldn't work. However, if you are travelling as a family or group then it is pretty likely you will get one of every seat (i.e window, middle, aisle) and so can choose amongst your group. If you are travelling alone then it is just £3 to choose your seat between rows 7-26 (A319) and 8-31 (A320) excluding the overwing exits.
Even for a family of 4 under a tight budget, in a difficult economic climate, £12 (per sector) is pretty good value compared to other carriers. I paid 20AUD for a single selected seat on QF.