when I intent to re-enter to the USA on the same trip (happens to me when I make detours to Vancouver Whistler for skiing, or to Mexico-City for discussing and buying Tequila) I make sure to keep the exit-paper that immigration did attach to my passport when entering the first time - never a problem when entering the second time (when crossing the Canada-USA boarder I just showed my passport, with the attached paper, sitting in my car, and was waived through (last time in 2002).