All of the fuel is useable on the A330/A340. Fuel is initially transferred aft, but small quantities are then pumped forward throughout the flight to keep the CG close to the aft CG target as fuel is used from the centre/inner tanks. Any remaining trim tank fuel is transferred forward towards the end of the flight.
Except when the main pumps are off and 2,000kg in each inner tank becomes unusable. This has led to some interesting outcomes in the simulator when dealing with low fuel level ECAMS where pumps are switched off and then attitude changes in go-arounds etc have then generated pump low pressure ECAMS.