The idea that the saved go to heaven when they die is somewhat anachronistic. Equally anachronistic is the idea that people will be "raptured" into heaven when Jesus returns. Paul had another meaning in mind when he wrote this verse.
N. T. Wright explains in Surprised By Hope:
The word parousia occurs in two of the key passages [concerning rapture theology] in Paul (I Thessalonians 4:15 and I Corinthians 15:23), and is found frequently elsewhere in Paul and the New Testament. It seems clear that the early Christians knew the work well, and knew what was meant by it. People often assume that the early chruch used parousia simply to mean "the second coming of Jesus" and that by this event they all envisioned, in a quite literal fashion, the scenario of I Thessalonians 4:16-17 (Jesus coming down on a cloud and people flying upward to meet him). Neither of these assumptions is in fact correct.—p. 128
Wright then explains the two meanings of parousia that were in play:
"The mysterious presence of a god or divinity, particularly when the power of this god was revealed in healing." (p. 129)
"When a person of high rank makes a visit to a subject state, particularly when a king or emperor visits a colony or province." (p. 129)
As Wright explains it, Paul envisioned Jesus returning to earth to establish His everlasting Kingdom here, not in heaven. Therefore, the spirits of the already dead will return to inhabit their resurrected bodies and the bodies of the living will be transformed so that we can meet our King.