TreeHugger did the math, and came up with some pretty interesting results. It turns out that pressing the cold/cold button (instead of the hot/warm button) on your washing machine has the same impact as...
...driving about 9 miles in a car or the production, transportation and storage of a six pack of beer. It may not be too surprising that one load of laundry doesn't make a huge amount of difference compared to, say, not eating meat or dairy. But, multiply those impacts by 392 -- the number of laundry loads an average U.S. home washes in a year -- and, all of the sudden, there are some real impacts.
Washing laundry in hot water is really wasteful
To wit: Washing every load on the hot/warm cycle (in a top loading machine and an electric water heater) for a year is equivalent to burning about 182 gallons of gasoline in a car; in an average (19.8 miles per gallon) car, that'll get you around 3595 miles. So, wash in hot/warm, or drive almost 3600 miles -- same difference.
Similarly, if you wash with the hot/cold cycle (in a top loading machine and an electric water heater), you'll end up with 2407 pounds of CO2 per year -- just over a metric ton -- which is equal to about one round-trip cross-country flight (6171 miles of long-haul flying).
What does this all mean? Aside from being a great example of how little decisions add up to make a big difference, it shows how wasteful heating large quantities of water can be. Just selecting the "cold/cold" cycle has the potential to save as much CO2 emissions each year as thousands of miles driven in a car, or even an airplane flight or two.