I suppose the best thing to do is simply ask before you buy?
If everyone just said "no" to buying locked phones, they'd have to stop locking them?
I think it's a p*ss-take to lock them anyway, especially if it was supplied with a contract. It's like buying a car and no-one else being allowed to drive it unless they have the same name as you!
Yeah I know the networks subsidise the price and therefore lock them so that they will get some revenue back, but since unlocking isn't illegal and can be done for a little as £5, it's hardly a foolproof way of getting some of the money back is it?