I've changed "timestamp" to "timestamptz" and have begun fixing some code that used to be legal but as of PG 7.3 is not. You can no longer subtract an integer from a timestamp but must cast to date, "timestamp" disappeared (must use "interval" now), "datetime" disappeared ("timestamptz"), and suchwhat.
Added "last_post" column to the forums table and "last_child_post" column to the forums messages table to track these values. Previously the code was abusing the related object's last_modified field and it didn't bother tracking the date of the last reply to a thread.