Did you know that the average mobile phone consumes 2.8 KW of electricity per year? Since iPhones and Blackberry's are running so many applications and functions, they are charged more often than the regular old cell phone, typically every 24 to 48 hours. I'm sure the Palm Pre and mobile platforms with the Android OS will be very similar. Unless you live in a city with abundant hydropower, electricity will be one of the main contributing causes of greenhouse gas emissions... and it is expensive!
Do you ever plug in your iPhone, or any battery powered device for that matter, and let it charge all night so it has a full charge when you leave for work in the morning? Doing this is basically overcharging the battery and wasting electricity. At least the new iPhone 3GS has the built in display showing battery percentage, which is much improved over the application designed to do that for the 3G, which doesn't show it.
Impetus Technologies has designed a new iPhone app that might just save you some money on your electricity bill and help the environment along the way. Green Charging alerts the user (with sound effects and vibration) when the iPhone's battery is fully charged. As a bonus, the application also shows battery status, stand-by time and talk-time, in percent. The 99 cents it costs to buy this app will definitely pay for itself many times over the life of the phone.
This might sound too basic but another tip is to turn your mobile device off occasionally. I have a friend who had his phone for over a month and kept it on the whole time. He charged it at night while the phone was still on. When the iPhone's battery finally died one day, he couldn't remember how to turn it back on after he had it on the charger.