And I wish I knew where I saw it, but there was a site that had a pretty darn good story of how they did Paul Stanley's cracked mirror guitar back in the 70's.
The final variation ended up being that they routed the top of the guitar down (to match the thickness of the mirror), but left the binding on to work as a final, clean edge.
They tried smashing the mirror, but that just turned into a mess. The best result came from designing a crack-pattern, and then cutting it that way. I believe they also added some type of clear material on top, to avoid getting cut by the mirror edges. You would think that with the mirror pieces flat, and pushed together, there is no way you can cut yourself...
Well, they didn't just leave the pieces flat, but "angled" them all slightly to really get maximum reflections from the guitar. If you just put the mirror back together, perfectly flat, it ends up barely looking cracked, and just giving off one large reflection...
You could always do it like this:
http://www.vai.com/Machines/guitarpages/guitar037.html