Who are the "kings of the earth?"

I don't think that reference to "the ruler of the kings of the earth" has anything to do with us being a "kingdom of priests" though obviously Jesus has authority over us.  I think rather, the reference is to what we normally think of as kings -- and by extension, the heads of other types of governments.  I would restate the phrase, "above all earthly power."

One idea that we considered in response to this, is that Revelation was written after Christ's death.  The instance you mention in Luke was before his death.  Christs death on the cross was for the purpose of purchasing all men, right?  Which would include the kings of the earth.  Acts 20:28 says,

"Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood."

Interestingly though, Psalms 74:2 says similarly:

"Remember thy congregation, which thou hast purchased of old; the rod of thine inheritance, which thou hast redeemed; this mount Zion, wherein thou hast dwelt." 

See also Exodus 15:16

This same reference being used prior to Christs death might disprove this theory, but perhaps these older references only refer to Israel.  Then again, Israel is spiritually all of God's people.

I think I agree with what you mentioned to me the other day: that this verse is saying that Jesus is the real king -- the legitimate king --  with no implication that the kings of the earth are his vassals.