superuser.apk is the front end on the phone, su is the actual 'elevation' to root program which if I am not mistaken is a hardlink to a patched busybox(with su compiled in) so technically you cannot 'unroot' without replacing the busybox program by the stock one.
but as mentioned above, rooting makes no harm to the system in the tinest way. if you are afraid of returning the phone, the sales in the shop is not sophisticated enough to know the difference.