The only version of W8 that is available at the moment is the Developer Preview, hardly usable, but you can install it the same way you install any other ISO onto a memory stick
Download Windows 7 USB tool and that will do the job for you, just point it at the W8 ISO
As for the hardly usable comment above thats nonsense, I have been running it since September on my Mac with no problems.
I don't think so, what I thought you meant was to install Windows 8 via a USB, bootcamp should then give you the option of booting into windows once you've installed it.
I would assume that when you try to install Windows 8 via USB bootcamp would pick it up the ISO from wherever it is and let you install it