There's a restroom at a (hotel, I thnk) here that the wall behind the urinal is made of one way glass - so you'll be standing there going to the toilet and be able to see everyone outside eating... someone I know was there once an every so often pointed at the glass and laughed, just to embarrass the people behind it (he couldn't see through, but you'd think he could.)
I've also heard of a cubicle that is made of glass, which frosts over when you latch the door.