We used the Virgin "check in n chill out" system on our return from Florida. It was never mentioned that our suitcases should not be locked on departing the US. Out of our six suitcases, one was opened by "the officials", breaking the lock in the process. Luckily this case had a strap tied round it so it was able to be kept closed. Inside the case we found a note saying the Officials are entitled to open the cases and if they break the case - hard luck. They do not accept responsibility for repairing the damage.
BE WARNED. I think Virgin should have mentioned this to us when we handed out suitcases over to them!