Available at AT&T stores starting in May (for an unknown price), the AT&T Zero Charger is the world's first wall-based USB charger that draws absolutely no power when it's not refilling a cellphone. Now follow suit, everyone else. [AT&T]
UPDATE: It looks like another company beat AT&T to it and did things better. From Volt-Star:
It is our understanding that the AT&T device automatically shut down when the device or cell phone is unplugged. The VoltStar Eco Charger goes one step farther and shuts down once the phone is charged.