Create a new .PST file and copy the Contacts folder to it.  Close the new
.PST file, then copy it to the USB key.  To restore it later, copy it back to
a hard drive, make sure it's not marked read-only, and open it from *within*
Outlook using File | Open | Outlook Data File.  You might want to verify that
it works by going through this process once before relying on the file when
you don't have the original info on hand.

Please note that a Unicode format .PST -- the default used by Outlook 2007
and 2003 -- is not backwards compatible.  If you will need to use the
contacts on an earlier version of Outlook, when you create the new .PST file
to copy the contacts to, make sure it's in the old Outlook 97-2002 format.  
This format can be opened in any Outlook version.