This is a question of safety for children using the device while they are still being shipped without a case.
The USB power source is expected to provide 5V and at least 700mA. Is this enough to give a shock?
Do any components on the board step the voltage up, giving a higher voltage?
Capacitors certainly would have the capability of giving a sudden release of current that is briefly higher than the input current (I remember reading a few accounts of people being shocked by a standard PC after unplugging it, thanks to the capacitors in the PSU). Would this be a possibility too?
I guess what it comes down to is how safe a Raspberry Pi is for a child to be using, if it doesn't have a case.