This script does 90% of the work in reading a database schema into an OmniGraffle document. If we can get some AppleScript whizzes to help, we can go for 100% - currently you may need to send connection lines "to the back" and adjust layout a bit.
It relies upon appscript to interact with a live instance of OmniGraffle (be sure to read the Python install info for some non-standard build instructions on Snow Leopard).
Originally ported from a script written by Paul Davis (see http://overooped.com/post/89728630/dbgraffle4-automatically-draw-sql-schema-in for some background).
The current script is available as an application at https://bitbucket.org/zzzeek/schema_to_graffle.