Just had my windows tinted (front/rear and back)... I know the manual warns against this with the Nav antenna being located somewhere beneath the rear window... but I haven't had one problem with the XM or Nav since the tint-job. If anyone wants a great tint that won't cause problems... I went with LLUMAR. It's non-metalic, and carries a lifetime warranty against fading, cracking, peeling.
Goodluck. I was worried about doing it because of what the manual said.. so I thought I would share.. in case anyone else is thinking of doing it.

-NavyDoc

 

Non-metallic tints are fine for our car. Ceramic based tints are the best!

 

I agree... the Ceramic's are awesome... and they keep out the most heat... but man are they $$$