Hmm...the latest BootCamp Assistant - at least, the one which comes pre-installed on the rMBP (not sure if my subsequent update to 10.8 bumped the version, but what I have installed now reports v5.0.0 (444) ) will take an installation DVD or an ISO image and write everything necessary to a USB disk.
I can verify that this works, since I used it myself with Win7.