If you need occasional access to Windows and don’t want to install it from a DVD, a how-to guide by Sam Power may have the perfect solution. It talks you through exactly how to create a Bootcamp-compatible USB key installer for either Windows 7 or 8 compatible.
A 4GB USB key is sufficient, and the complete process can be done in 20-40 minutes.
(Update: sorry for the confusion. You can’t install Bootcamp onto a USB Key yet. You’ll need Paralllels or VWMware for that.)