What does it mean to be "in the Spirit?"

All the references, in Revelation (Rev 1:10, 4:2, 17:3, 21:10), to being "in the Spirit" are immediately followed by an unearthly experience.  There appears to be a difference between the first instance and all the others though.  All instances except the first are followed by John being taken somewhere.

John being taken in the Spirit multiple times, makes it sound like Revelation may have been revealed in several visions.

In 1 Samuel 10:6, Samuel, prohecying about Saul says, "... the spirit of the Lord will come upon thee, and thou shalt prophesy with them, and shalt be turned into another man."

But, is being "in the spirit" the same as having the "spriti of the Lord come upon you?"  In order to prohecy, does the spirit have to come on you?

Joel 2:28 says, ".. it shall come to pass afterward, that I will pur out my spirit upon all flesh; and you sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions."

In Luke 4:18 and 19, Jesus declars, "The Spirit of the Lord is upon me, because he hath anounted me to preach the gospel to the poor; he hath sent me to heal the brokenhearted, to preach dilverance to the captives, and recovering of sight to the blind, to set at liberty them that are bruised, to preach the acceptable year of the Lord."