Can you recharge a laptop via a USB connection?

While everyone is correct to say, "no," let me give you the reason why:

Charging a laptop, even a netbook requires about 40 watts to charge (give or take depending on the model). USB can only output 5 watts. Even the special iPad charger only outputs 10 watts. Plus, the circuitry is not designed such that the USB circuits can accept INCOMING power.