A few scientific studies have been done showing that a dishwasher is in fact more efficient not only from a water usage perspective, but also from an energy consumption perspective. When you factor in the additional time that it frees up to spend time with the family, or do other more enjoyable tasks, it has always been an easy decision for me to utilize the dishwasher. Here is an interesting article that goes into some of the considerations in analyzing this, with some ranges of numbers and a brief hypothetical analysis included.
http://www.treehugger.com/kitchen-design/built-in-dishwashers-vs-hand-washing-which-is-greener.html
I've actually looked into this some, and as far as I can find there has only been one study done - the University of Bonn study linked from that Treehugger article. Every other article I've ever found referenced that single study. It's not a bad study, but it has some caveats. Prepare for the
Wall of Text.
The study is based on cleaning 12 place settings. This is perfect for a dishwasher, as plates, glasses & silverware tend to fit very well. But unless you just served carryout to 12 people, it's an incomplete picture. Where are the pots, pans, cutting boards, mixing bowls, chef's knives, and collanders that were used to prepare that meal? If you cook your own food those items need to be washed too, but they usually don't fit well in a dishwasher. You might need to run the dishwasher time (or two or three!) to clean those items, while they might be very easy to clean by hand. (As an aside, some items such as sharp knives and wooden items can be damaged by a dishwasher. Clearly destroying a tool is inefficient, so they should be handwashed in any case.)
Second, they used a random sampling of people to do the handwashing. It's a reasonable choice, but some of the people were terribly inefficient. For example, many of the people used 40-60 litres of water, but one person managed to use 447 litres! He/she literally left the hot water running full speed during the entire washing
and drying process. So while the numbers may represent a population average, it seems obvious that even a modest attempt at efficiency could do much better. (Another aside: many of the people managed to not even clean the dishes well enough that they would be willing to use them. Clearly these were not skilled practitioners of the dishwashing arts!)
That said, I think there's an interesting point to be made about the minimum values observed. The minimum water used for hand washing was 20-30 litres. The dishwashers averaged 15 litres of water. So in the specific case of loads that can be densely packed in the dishwasher, it is probably more efficient to use the dishwasher than to hand wash. In the case of large or irregular items, I think the jury is still out.
My personal procedure is to put all the plates, bowls, glasses and silverware in the dishwasher. Everything else gets handwashed. I can't prove that this is the most efficient approach, but it seems at least competitive.