The back of course, unless anyone's managed to scratch the glass front 
No idea how but mine's already acquired a scuff mark and a few light scratches and I've been incredibly careful and kept it in a (PDA) case too. Think it must've got them from very carefully and gently placing it on the desk without some kind of microfiber mat to protect it
So yeah they're incredibly easy to scratch - if you want yours to stay spotless you'd better not put it down, or at least keep it in a case and never take it out. How did yours acquire its wounds?
No idea how but mine's already acquired a scuff mark and a few light scratches and I've been incredibly careful and kept it in a (PDA) case too. Think it must've got them from very carefully and gently placing it on the desk without some kind of microfiber mat to protect it