There are servicemen buried in the tomb, but the last interred, for the Vietnam war was removed and identified through DNA and reburied near his home, here is the article from Wikipedia
The Tomb of the Unknowns at Arlington National Cemetery is also known as the Tomb of the Unknown Soldier. It stands on top of a hill overlooking Washington, D.C.
One of the more popular sites at the Cemetery, the tomb is made from Yule marble quarried in Colorado. It consists of seven pieces, with a total weight of 79 short tons (72 metric tons). The tomb was completed and opened to the public April 9, 1932, at a cost of $48,000.
It was initially named the "Tomb of the Unknown Soldier." Other unknown servicemen were later entombed there, and it became known as the "Tomb of the Unknowns", though it has never been officially named. The soldiers entombed there are:
Unknown Soldier of World War I, interred November 11, 1921. President Warren G. Harding presided.
Unknown Soldier of World War II
, interred May 30, 1958. President Dwight D. Eisenhower presided.
Unknown Soldier of the Korean War, also interred May 30, 1958. President Dwight Eisenhower presided again, Vice President Richard Nixon acted as next of kin.
· Unknown Soldier of the Vietnam War, interred May 28, 1984. President Ronald Reagan presided. The remains of the Vietnam Unknown were disinterred, under the authority of President Bill Clinton, on May 14, 1998, and were identified as those of Air Force 1st Lt. Michael J. Blassie, whose family had him reinterred near their home in St. Louis, Missouri. It has been determined that the crypt at the Tomb of the Unknowns that contained the remains of the Vietnam Unknown will remain empty.
The Tomb of the Unknowns is perpetually guarded by the U.S. Army. The 3rd U.S. Infantry Regiment ("The Old Guard") began guarding the Tomb April 6, 1948.
With DNA tracking today, I doubt there will be another unknown soldier buried there.