The big-name dishwasher detergents contain high amounts of phosphates--ingredients that kill the oxygen in waterways, and promote algae growth. (A recent study by the state of Minnesota found that dish washing detergent makes up 2 percent of the phosphorus found in its waters.) Meanwhile, Consumer Reports has found that all-natural, biodegradeable dishwater detergents work just as well as the standard phosphate-filled ones.
Hypocrisy meter: I escape unscathed on this one. We use Seventh Generation all-natural dish detergent. It works great. No streaks or spots. (We buy a giant box of the stuff at Costco. As for the environmental impact of shopping at Costco, we'll save that for a later date.)