Here is a nice blog post by Blue Lyon, discussing the energy cost and money 'savings' offered by AT+T's recommendation to Turn your iPhone green with a new ZERO Charger.
Blue Lyon works out that the charger, which claims to use less power on standby, might pay for itself in 44 years, assuming it was displacing an old charger using 0.26 W, left plugged in all the time.
The makers of 'ZERO Chargers' are therefore nominated for the Hot Air Oscar for flagrant exploitation of gullible consumers.
One way to think about this is simply to look at the energy cost of delivery alone, assuming that the delivery involves one van making a 5-mile trip. Typing this into my firefox browser
5 miles / (13.1 miles per US gallon) * 10 kWh per litre in kWh
gives 14 kWh of transport energy to deliver the greenwart. That corresponds to the energy used by leaving the old charger in for 6 years.