Last night ( i just remembered), i was at my LFS and they where setting up a new 60 gallon tank, instead of a background glass it had a mirror!!!
Of course the first thin on my mind was that it looked cheesy... well i have to accept in spite of my hate for mirrors that it looked pretty cool, it gave the tank more depth and when they added the plants, it looked like a jungle in there quite interesting effect.
I still wouldn't try it myself, but it's an alternative