(FAQ?) JOIN condition - 'WHERE NULL = NULL'

This is probably a stupid question that has a very quick answer, however it
would be great if someone could put me out of my misery...

I'm trying to JOIN two tables (well a table and a resultset from a PL/pgsql
function) where a joining column can be NULL

-- for a given id, return a bunch of columns that I can use for joins

psql> select * from get_colnames_for_id('1.10.8');

depth1 | depth2 | depth3 | depth4 | depth5 |
--------+--------+--------+--------+--------+
1 | 10 | 8 | | |
(1 row)

--
psql> select * from table where depth1 = 1 AND depth2 = 10 AND depth3 = 8
AND depth4 IS NULL;

id | depth1 | depth2 | depth3 | depth4 | depth5 | name |
---+--------+--------+--------+--------+--------+----------------------+
1 | 1 | 10 | 8 | | | name for node 1.10.8 |

(1 row)

-- I (wrongly) expected the following to be equivalent

psql> select c.* from get_cathcode('1.10.8') c JOIN table t USING(depth1,
depth2, depth3, depth4);

id | depth1 | depth2 | depth3 | depth4 | depth5 | name |
---+--------+--------+--------+--------+--------+-----------+
(0 rows)

-- Whereas the following works...

psql> select * from get_colnames_for_id('1.10.8.10');

depth1 | depth2 | depth3 | depth4 | depth5 |
--------+--------+--------+--------+--------+
1 | 10 | 8 | 10 | |
(1 row)

psql> select c.* from get_cathcode('1.10.8.10') c JOIN table t USING(depth1,
depth2, depth3, depth4);

id | depth1 | depth2 | depth3 | depth4 | depth5 | name |
---+--------+--------+--------+--------+--------+-------------------------+
2 | 1 | 10 | 8 | 10 | | name for node 1.10.8.10 |
(0 rows)

So, I'm currently assuming from all this that joining on t1.col = t2.col
doesn't make any sense when t1.col and t2.col are both NULL - since:

psql> SELECT (NULL = NULL) IS TRUE;
?column?
----------
f
(1 row)

psql> SELECT (NULL IS NULL) IS TRUE;
?column?
----------
t
(1 row)

Unless I've missed something, the docs on
http://www.postgresql.org/docs/8.1/static/functions-comparison.html seem to
suggest that the concept is an example of bad programming and the workaround
(of switching on the 'transform_null_equals' config) is a hack. Is this all
true or did my logic just get screwed up at some point? Unless I've just
missed something obvious, it seems useful to be able to join two tables
based on a condition where they share a NULL column - is there another way
of doing this?

Many thanks,

Ian