If you look long and hard enough you can find Firewire thumb drives, Micromat uses one for their
Protege. But those puppies are not exactly cheap and their capacity is very limited. Micromat makes do with a special stripped down version of OS X on the Protege.
Knoodle's solution is the one I arrived at several months ago. I was upgrading the 160GB hard drive in a friend's older MacBook Pro with a shiny new 500GB drive. As my fee for the upgrade I kept the 160GB drive and mounted it in an OWC Mercury On The Go enclosure. Formatted it, installed OS X and a variety of disk and system repair utilities and it is now my general purpose
Emergency drive. With the help of a $15 powerbrick it will even boot my MacBook Air so I can run diagnostics and disk repairs if necessary.