Bootcamp over external USB drive
Boot Camp will not install to an external drive, (pg. 6) but you might be able to clone a Windows system to the external volume; not sure if a Mac will support booting it via USB 3.0 though. I'd say you should wait for the next Boot Camp update which will likely bring USB 3 support (and hopefully native drivers for all the new Mac models introduced at WWDC 2012).