Guillaume,
when you are using JON, you should contact JON support. JON 2.4 is based on RHQ 3 code which is delivered and tested with the Postgres 8 jdbc driver and postgres 8.2.4+ (but not 9.x). JON 2.4 is only supported those postgres 8.x versions.
Having said that, in RHQ the jdbc driver lives in jbossas/server/default/lib/postgresql-*.jar
Heiko