I am level 60 and am far into Inferno difficulty. I started to help my friend in Nightmare difficulty, when I started to get stacks of NV. Is this supposed to happen on non-inferno difficulties?
Nephalem Valor is availible to all level 60 characters, regardless of difficulty. You can even take it into Normal or Nightmare, if you prefer (and as you experienced).
Blizzard has stated that this is intended, the conventional wisdom being that some folks will need it to farm Act IV Hell before breaching Act I Inferno.