Note: The following applies not just to USB hard disks, but to any storage device that is not considered "internal". That includes USB flash drives, SD cards and other memory cards, as well as hard drives attached through Firewire or other connections.
Booting Windows or Linux from an external disk is not well-supported by Apple’s firmware. It may work for you, but if it does not work, there is nothing rEFIt can do about it.
Apple’s Boot Camp FAQ states that you can only install Windows on internal hard drives.
Note that at least Windows XP needs special patches to successfully boot from a USB or Firewire drive. That’s because it normally re-initializes the driver stack during the boot process in a way that disrupts its own access to the boot drive.