just got our iphone 5 yesterday... and we tried cutting our microsim into nanosim. confirmed 100% it can work, we tried with a umobile sim. didnt even need to sandpaper it as the sim tray of the iphone5 was still very loose and could be taken in and out easily
Thanks for posting. Is your u-mobile simcard the older type? Mine is and there is no more space to cut unless part of the contacts are cut away as well