Index on a NULL-value

I read in the manual today:

Indexes are not used for IS NULL clauses by default. The best way to use
indexes in such cases is to create a partial index using an IS NULL
predicate.

This is from the documentation for PostgreSQL 8. I did not find anything
equivalent in the 7.4.8-documentation.

I wasn't aware of this until it became an issue :-) Well, so I follow the
tip but in vain. Reduced and reproduced like this in PostgreSQL 7.4.7:

test=# create table mock(a int, b int);
CREATE TABLE
test=# create index b_is_null on mock((b IS NULL));
CREATE INDEX
test=# insert into mock values (10,20);
INSERT 70385040 1
test=# insert into mock values (20,30);
INSERT 70385041 1
test=# insert into mock values (30, NULL);
INSERT 70385042 1
test=# set enable_seqscan=off;
SET
test=# explain select * from mock where b is NULL;
QUERY PLAN
--------------------------------------------------------------------
Seq Scan on mock (cost=100000000.00..100000020.00 rows=6 width=8)
Filter: (b IS NULL)
(2 rows)

vacuum analyze also didn't help to recognize the index ;-)

Any tips? Rewrite the application to not use NULL-values? Hide under
bedclothes and hope the problem goes away? Install more memory in the
server? :-)

--
Tobias Brox, Beijing