BUG #7501: Unique index fails to check uniqueness on nulls

The following bug has been logged on the website:

Bug reference: 7501
Logged by: Matt Kusnierz
Email address: matthew(dot)p(dot)kusnierz(at)jpmorgan(dot)com
PostgreSQL version: 9.0.4
Operating system: Windows XP
Description:

Multi-column unique indexes allow duplicate rows to be inserted if any of
the indexed columns are inserted with a null.

For example:

create table foo (a varchar(10) null, b varchar(10) null);
create unique index uix on foo (a, b);

insert into foo (a, b) values ('abc', null);
insert into foo (a, b) values ('abc', null);

Clearly the second insert should fail with an ERROR: duplicate key value
violates unique constraint "uix"

But this does not occur!