I've added these lines to postgres.conf:
dynamic_library_path =
'$libdir:/var/lib/pgsql/pljava:/usr/java/j2sdk1.4.1_02/jre/lib/i386/client/'
custom_variable_classes = 'pljava'
pljava.classpath =
'/var/lib/pgsql/pljava/pljava.jar:/var/lib/pgsql/pljava:/usr/java/j2sdk1.4.1
_02/jre/lib/i386/client/'
pljava.release_lingering_savepoints = true
pljava.vmoptions = '-Xmx64M -Dbackchannel.port=48'