If you buy online, call Apple and demand for one-to-one exchange. According to my understanding, Apple will ask DHL come and collect from you then they will send you a new one within few days. If you bring back to local service provider, most likely you will get a refurbished set since they will get the replacement from Apple instead of one-to-one exchange for you on the spot.

I saw ppl posted in iphone thread. Those purchased from Apple online have called back Apple toll free and they have their iphone collected by DHL.

Is the spot on the screen very noticeable? which part of the screen?

I believe either send back to Apple or service provider you also have to wait. The only way you do not need to wait is to leave with it if it is not so noticeable.