these replacement units from the store, according to a senior tech at apple care, could come from anywhere. they dont necessarily come 'refurbished' or 'remanufactured' from the factory. i have gone through 4 different replacements before calling the senior tech at apple care to send me a brand new one in box (which i am waiting for right now). i actually got a replacement iphone 3gs from the store with a small piece of an invisible shield still left on it, and some adhesive remnants from it still along the edges of the phone. funny thing is, the genius who gave it to me told me that the whole outer casing and almost all of the inside parts were brand new! HAHA wat a lie. AND, with each replacement, they got worse and worse. all had the rattle problem (which ive grown to accept), most had accelerometers being 5+ degrees off, most had uneven glass screens not being flush with the chrome bezel, and ALL had serious gps issues (only received triangulation signal...MUCH worse compared to my first brand new iphone 3gs. and yes, i exhausted all tests/options, even restores!). looking back, the FIRST iphone 3gs i had, which was the one i received brand new in box, was in the best shape of all 5 that i got to try (the only issue with that one was the accelerometer was off by 5 degrees. annoying when playing racing games.)
so my conclusion after my experiences with refurbs and my talk with the senior tech: dont rely on the apple store replacements. im sure they have some good ones in there, but i know im not the only one whos received an 'unlucky' streak of 4+ bad replacement iphones. after much research, i feel my situation is not an isolated issue, and i still find it very hard to believe that these replacement units have gone through extensive QA and are in "like new" condition. i really wouldnt be surprised if they just took iphones that were failing and gave them away as replacements to other customers without even looking at them much.
heres hoping that my NEW brand new in box iphone 3gs will be perfect!
goodluck!!