We're asking for a God who lives in heaven to set foot on the ground. We're asking for this world of blood and bodies to be transformed, essentially into another world. It's not that heaven is just some pie-in-the-sky world of spirits and angels. And it's not that God is hanging out in some other world, like on another planet, thinking about whether or not to show up in this one.
To the contrary, God is already in this world even as we wait upon the Spirit. The Spirit of the living God is the Spirit of a God who's embodied, quite distinctly, in human flesh and blood. God is with us in this world from which we cry. And, to the contrary, heaven is not some floating, nebulous, ether that can't be bothered with dirt and water and plants and bread. Heaven is quite like earth. Heaven, if you dare to believe it, is just the way the earth is actually headed, it's our future; a future heralded by a resurrection; a resurrection that doesn't forget about bodies and brokenness but finds them living where we'd only expect them to be dead.
When we say that God is in heaven, we say that God is where we're going. When we call on the Spirit of the Living God and when we cry for it to be "on earth as it is in heaven," we are implying the audacious claim that there is hope; hope for the dignity of life and the humanity of humanity to be restored. And when we witness to that hope, when we whisper such absurdity by saying simple things like, "peace be with you", we do a very funny thing.