I care about the environment, but I use warm (if not hot) for undies, sheets and towels, because I just am not sure about how clean they will be otherwise! Most other items I don't mind washing in cold water, being sure to separate whites from darks.
Just for fun, you could try washing everything (or everything you dare) in cold water for a month, and see if there's a difference in your utility bill. That can give you some idea how much energy you're saving as well.
This week I started trying out homemade laundry detergent, so I can't give an evaluation of it yet - except that it dissolves in cold water as well as warm (the stuff starts out as a slippery-looking gel), which is more than commercially-made powdered detergent does for me!