Have you ever wondered if washing your clothes in hot water really mattered? I can tell you, unless you have heavily soiled clothing, it doesn't. The Farmer and I wash almost all of our clothing on the cold setting and still have clean clothes. Your cold water setting and the power of your detergent will do the work helping you save money and energy.
Your washing machine uses most of it's energy on heating the hot water cycle. So if a regular batch of laundry (not heavily soiled) is washed in hot water, energy is actually being wasted. Using less hot water (and saving energy) can lower greenhouse gas emissions (which is what we're really after).
I can also tell you from personal experience that the less work your water heater has to do, the less you are going to pay on your energy bill. The Farmer and I learned this last year when our water heater broke and was running constantly...causing our energy bill to sky rocket (over $120 dollars more than normal). I'm not saying you have to bathe in cold water, but it wouldn't hurt to use it on your clothes.