Are there any Mining OS to be started from USB?
LinuxCoin is probably the closest thing to what you're looking for. According to the wiki, Linuxcoin is:


  A lightweight Debian-based OS which includes in the image everything
  that is necessary to run as a Bitcoin client and everything needed to
  get started mining on ATI hardware.


Edit: It may also be worth looking into BAMT if your operation is of significant size

Update: How things change... BAMT is now the de facto mining distro. I'm now having a difficult time even finding a live link to download LinuxCoin from and BAMT has gone from experimental to a full-fledged mining distro usable for nearly any scale. I highly recommend BAMT over LinuxCoin.