The National Park Service website says that Grant's Tomb does, in fact, contain the bodies of President and Mrs. Grant. Why the mix-up? Well, the old joke is "Who's buried in Grant's Tomb?" and the answer is "no one." Grant's tomb is a mausoleum (North America's largest, 150 feet high), which means that the bodies are contained above ground. In order to be buried, you have to rest underneath Mother Earth herself. So, nobody's buried in Grant's Tomb, but the remains of the Grants are there anyway. Ain't America grand?
http://www.nps.gov/gegr/