Boot Camp on Late 2010 Macbook Air using USB flash drive?
Bootcamp is a tool for installing Windows on the same drive with your OS X.

If you want to have Windows on a USB, then boot from your Windows installation cd and install Windows on the USB.

Then whenever you want to boot into Windows, plug your USB, hit alt/option and select Windows as your boot disc.

It may be better though, if you find a ready/made Windows USB Edition.

Either way, it will work :)

Edit:

After your first boot into Windows, do not forget to insert the Mac OS X Installation disc, in order to install the Windows/Bootcamp drivers!