Does this mean that you can leave your standard AC adapter at home? Not exactly. When I plug in a micro USB charger to the ThinkPad Tablet 2 or Latitude 10, you'll see that Windows says "plugged in, not charging" when the display is on. Turn the screen or device off and the battery will charge slowly.
Now, let's get technical:
USB provides 5 volts (5V). USB chargers range from less than 0.5 amps (500 mAh) to 2 amps (2A). Most laptops give their USB ports 0.5 A of power.
Standalone USB chargers vary in amperage. For example, the iPad charger is a 5V, 2A charger. That calculates to be a 10 Watt charger (5V x 2A = 10 W).
An iPhone charger is a 5V, 1A charger. That calculates to be a 5 Watt charger (5V x 1A = 5W). That's why it's such a tiny little charger.
Most tablet PCs require more than 5 volts to operate. The Dell Latitude 10 has a removable battery and you'll see that the battery is rated at 7.4 volts. So, there's no way a USB charger rated at 5V can keep the tablet on and also charge your battery.
Fortunately, most of the Intel Atom powered Windows 8 tablets have very long battery life (approx 10 hrs), so you can go on battery power during the day and charge them at night when they're off. The battery on the Dell Latitude 10 is removable and you can even get an extended battery that will give you 20 hrs of battery life. The HP ElitePad 900 comes with an optional Expansion Jacket that can house a removable battery, so you can also get 20 hrs of battery life with that configuration.