Peaceful rooms are meant to give you some place you can hang out and chat 
with friends (or enemies) while being able to let your guard down for a bit.

What this means specifically is that you can't be attacked by either other
players or mobs unless one of the following is true:

- You have attacked someone recently or they have attacked you and the
  victim hasn't forgiven the attacker.  This person or mob will be able to
  attack you again.

- You are in combat.  Once you are fighting, anyone can attack you.

- You have been hanging out in the room and blocking regens for quite a
  while.  Anyone can kill someone who has been camping in the room.

See Also: Forgive