8.1 Unique Index Issue/Bug???

I am having what appears to be a bug with unique indexes on 8.1.3.

I have created a new table.

create table payer_835 (
payer_id int8 not null default
nextval('payer_835_payer_id_seq'::regclass) primary key,
payer_name varchar(50) not null,
payer_trn03 varchar(10) not null,
payer_trn04 varchar(30),
sku_id int8 references skucode(sku_id) on delete cascade on
update cascade,
payer_billable boolean not null default true,
create_timestamp timestamp not null default now(),
last_mod_timestamp timestamp,
expire_timestamp timestamp
);

On this table, I have created a unique index on payer_trn03, payer_trn04,
and expire_timestamp. However, since the expire_timestamp is normally null,
the unique index does not appear to be working. I have been able to enter
two identical rows into this table.

Why is PostgreSQL not enforcing this index? This appears to be a pretty
major a bug? It would seem that you could have a unique index across
columns that might have a null in them.

Here is the data from the table:

COPY payer_835 (payer_id, payer_name, payer_trn03, payer_trn04, sku_id,
payer_billable, create_timestamp, last_mod_timestamp, expire_timestamp) FROM
stdin;
1 CAHABA GBA-AL PART B 1630103830 \N 1 t
2006-07-13 09:57:52.834631 \N \N
2 FEP 123456789 00402 1 t 2006-07-10
10:56:23 \N \N
3 NC Medicaid 123123123 \N 1 t 2006-07-10
10:56:41 \N \N
4 CAHABA GBA-AL PART B 1630103830 \N 1 t
2006-07-11 16:13:43.808181 2006-07-12 10:09:46.066204 \N
\.

Notice records 1 and 4 have identical data (as far as my unique index is
concerned), and the index it not complaining.

thanks,

Chris
RHAS 4.0
PG 8.1.3 (from rpms)