Booting from a USB is NOT possible, as previously mentioned. However running from a USB should be possible.
I haven't tried, but if you install LE on a USB stick and also a SD. Then on the SD card, change cmdline.txt for root= point to say sda1 (Many options are available LABEL, UUID) instead of mmcblk0p2. (This was done from memory, so may not be precisely correct)
Have you looked on the Wiki - I'm sure there might be an entry for this.