I thought this would clear up most of the misunderstandings.

As you probably already know chargers consist primarily of a simple transformer. Transformers have two independent circuits which are inductively coupled. Wiki for more info. A current through one coil will induce a current on another. Current is based on what resistive network it sees on the other coil (and of course itself). Under ideal conditions if there is no load (meaning resistance is ideally infinite) on the other coil no current will flow on the left side. In real life nothing is ever ideal, thus we need to get past the electrical engineering 101 ideology. It is possible for transformers to be no more than 85% efficient even without a load. The best chargers have been capable to reach 98.75% efficient. Also, further wear on the core and coils due to regular use will slowly reduce this number.

Now for pico-watts... that is a bit of an exaggeration. If a transistor (BJT) off power leakage is in area of micro-watts (considering 2v supply) I would assume a transformer (with a 120v) be in the milli-watt or watt area. this is practically negligible considering your meter on your house measures in kilos. It isn't even close to having a light bulb on 24/7. Unless you notice a big jump in your bill I wouldn't really worry about it. For the green people... their thought is that if a million people stop leaving there chargers hooked up then we would save 1 million x 1 milli-watt= 1 kilo-watt. WooHoo!!

easy check. If your charger is warm without a load, this is bad. Another way is to to test with an amp meter. I think I am going to go test this myself to find whether my proposal is correct.