How do I boot from a Windows USB with a Mac? I'm trying to get my Mac to boot from a USB flash drive. I know the drive works and is bootable, because it works with my friend's Windows PC.

Specifically, I was running Windows 7 on my Mac to create the bootable disk, which has Windows Defender Offline on it. I have had the same issue trying to use the Ultimate Boot CD on a USB, various Live USBs for Linux, etc.

So, what do I have to do to get my Mac to recognize and boot from another OS on a USB flash drive?

Or maybe a better question, if there isn't a simple answer, would be what does Bootcamp do to allow booting from a Windows install DVD or to allow Windows, once installed, to boot?

EDIT: I was able to sort of get results using the bless command in Terminal. Specifically, I used 

sudo bless -mount "/Volumes/WDO_Media32/" -legacy -setBoot -nextonly

and then did a restart. Boot was slower, but then I was presented with the normal log-in screen. The log in screen, though, was much slower than usual, as if something else was happening in the background. So, I seem to be effecting something with that command, but I still can't boot from my Windows bootable USB drive.
Boot Camp has to emulate a BIOS system, as Windows requires this to run.  Apple uses something called the bless tool in order to instruct EFI (which Macs are based on) to activate this BIOS system.

However, if I recall correctly, emulation of BIOS is not supported on Mac for any device that is not attached via a SATA connection (ie. internal drive, DVD drive).  So, I believe that you cannot boot from your USB key.

If you want, you could install an alternative bootloader on your Mac that would allow you to do this.  See this post for more info.