Unique index with Null value in one field

Hi,

I need to define a Unique index on 3 non-PK fields (composite key) on my
table in PostgreSQL 8.0.3.

The problem is, if any of those 3 fields is Null, PostgreSQL allows
duplicate rows to be inserted. While searching through archives, I found
more information about this.

But I need to know how can I make PostgreSQL throw error on attempt to
insert second record having same 3 field values, one of them being Null.

------------------------------------------------
myid | field1 | field2 | field3 | description
PK | <--- Unique Index ---> |
------------------------------------------------
100 | ABC | XYZ | <null> | Record 1 -> This is ok.
101 | ABC | XYZ | <null> | Record 2 -> * This should error!
------------------------------------------------

Fields {field1, field2, field3} have unique index on them and "myid" is
the primary key of my table.

Oracle 9i throws exception in such case, but PostgreSQL does not.

Thanks,
- Hrishi Joshi.