I used to have my desktop Windows XP pc go to standby (S3 suspend, I think?) when it idled for over an hour or so. Now I charge my iPhone 3GS from a USB port, often right when I go to bed. Ideally I'd like to save electricity by having the computer go to standby as soon as the phone is done charging.
Is this possible? Is it a pipe dream? I am open to whatever crazy solutions might work, including jailbreak apps working over my wifi network.
Edit: The reason I don't use the included charger is that I want to avoid buying another piece of junk that will eventually hit the landfill. Rather than buying car chargers for everything, I have a power inverter in my car, and the AC charger stays there. (I did buy an extra data cable.) So I'm looking for a solution that does not involve buying any hardware (nor moving my wall charger around every day).
Also, the iPhone 3GS won't charge from most USB chargers such as the ones linked in the answers so far.