While the rainbow has become a symbol of gays in the social world, it's symbology in the spiritual world is different.  Primarily, the rainbow symbolizes transformation, specifically, spiritual transformation.  It can also represent higher forms of being, higher levels of consciousness, and the bridge between Heaven and Earth or between the material and the spiritual.  It can also symbolize, in some areas, a serpent that drinks from the sea.  In parts of Africa, the rainbow is a symbol of the celestial serpent that is a guardian of heavenly treasures.  It can represent the energy centers (chakras).  In Hinduism, the "rainbow body" is the highest yogic state of being in the material world (samsara).  Of course, we have all heard of following the rainbow to get the treasure, or pot of gold at the end.  This refers to wakening and energizing all the energy centers to achieve the great treasure of spiritual enlightenment.
    There is also the rainbow that appeared to Noah as a promise from God that the world would not be destroyed by flood again.
    Last century, a Cree wise woman foretold a future when the white mans greed would cause great destruction on the planet and the "keepers of legends" wold be called upon to bring the world back into balance.  These people were titled "Worriers of the Rainbow".  The Hopi, Sioux and Cherokee have a similar prophecy.  
    The Indalo is an ancient symbol found painted in a cave in Spain.  It shows a stick-man holding a rainbow.  The people there consider it a symbol of good luck and sometimes paint it over the door.  Archaeologists believe it was probably a symbol of God in an ancient religion.