Easy way to find out exact answer and stop the speculation is to look at the datasheet of the USB (power) controller. In another post it has been mentioned (N800) to be Texas Instruments part TPS65030.

http://focus.ti.com/lit/ds/symlink/tps65030.pdf

In the TPS65030 datasheet they mention absolute maximum current 200mA for the Vbus output. Above this point "permanent damage may occur".

They also say that the Vbus output is current limited, with the typical current limit being at 160mA. So, if you try to overload the USB port you should see the output voltage dropping or getting turned off around 160mA point on typical device. (YMMV) Also the absolute maximum short circuit current is less than 325mA.

And that's not all yet.. (as you have heard in TV-shop... ) The chip is overtemperature protected that in case you manage to overload it up to a point of failure the chip will turn itself off before that happens.

In short the output is protected against overload and short circuit. You shouldn't be able to fry the N800 by overloading or short circuiting the USB port +5V bus.

(This is common design practise that anyting that is user accessible you should assume user can short circuit those pins/wires and the hardware should not get damaged.)