There are already such devices on the market. I just bought one three weeks ago for about $12. It even has a plug for car cigarette lighter (which is actually why I bought it).
It's called "3-in-1 charger" here in Europe and it looks like this:
What the GP is talking about is a little female-microusb to male-microusb that disables the data pins so you're sure the only thing going on there is charging.
It's called "3-in-1 charger" here in Europe and it looks like this:
http://mireltech.ru/node/614