Postgres 7.1 and higher use a recovery log, aka write ahead log. Changes to the database are written to this logfile, then once a transaction is completed, the logfile has that change removed, IIRC.
The idea is that by simply replaying whatever is in the write ahead log in the event of a crash, all changes can be properly recorded, and the database can be back in production mode that much quicker.
I doubt that using a journaling fs would change much. I believe that PG no longer does an fsync to disk on every transaction as of 7.1.
Oracle and DB2 at least, offer the option to use "raw" devices, where you have a disk or part of a disk that does not have any filesystem on it. The DBMS has exclusive control over it.
