I managed to install Windows 8 RTM over the Windows 8 release preview bootcamp installation by simply downloading the ISO, mounting it, and opening Setup.exe in Windows.
After that, I booted back into OSX and used the Boot Camp Assistant to download the necessary drivers to a USB flashdrive. (I would have used Windows update, but the WIFI drivers were not working).
Then installed Bootcamp on Win 8 and everything is good.
I tried booting from a USB drive based on the ISO, but wasn't able to boot from it. (I held down the option key while restarting, but the USB device didn't show up).
I also tried booting from a burned copy of the ISO, but wasn't able to boot from that either.