Up to a point. There is an over-voltage diode but if your supply does not shut down, but instead puts enough energy through it, it will eventually blow up. Shortly after that you will have a baked Pi.
So, if the diode blows up then it will be shorted out?
I am less worried about the RPI than a $2000 TV getting fried by an overvolted HDMI port.
There is no way to make it instead disconnect the circuit?
Here's my understanding from the preliminary version of the power supply schematic. (There's a link to this schematic at the RasPi wiki: http://elinux.org/RPi_Hardware
If the supply voltage goes over 5V, zener diode D17 quickly clamps it to 5V and if the resulting current exceeds 1A then self-resetting fuse F3 opens the circuit. So there's a reasonable chance the RasPi and attached 5V equipment will survive. However, my understanding is that zener diodes are not very precise over a wide temperature range and fuses take a while to blow so I wouldn't like to depend on this circuit for everyday use.
You are much better off using a switching power supply to convert 6V or higher battery voltage to precision 5V.