uniqueness constraint with NULLs

I have a table with a uniqueness constraint on three columns:

# \d bobtest                         Table "public.bobtest" Column |  Type   |                      Modifiers
--------+---------+------------------------------------------------------ id     | integer | not null default
nextval('bobtest_id_seq'::regclass)a      | integer | b      | integer | c      | integer |
 
Indexes:    "bobtest_id_key" UNIQUE, btree (id)    "bobtest_unique" UNIQUE, btree (a, b, c)

I can insert multiple rows with identical a and b when c is NULL:
...
# insert into bobtest (a, b) values (1, 4);
INSERT 0 1
# insert into bobtest (a, b, c) values (1, 4, NULL);
INSERT 0 1
# select * from bobtest; id | a | b | c
----+---+---+---  1 | 1 | 2 | 1  2 | 1 | 3 | 1  3 | 1 | 4 | 1  4 | 1 | 4 |  5 | 1 | 4 |  6 | 1 | 4 |
(6 rows)

Can anyone suggest a way that I can impose uniqueness on a and b when
c is NULL?

In the real app., c is a date field and I require it to be NULL for
some rows. In these cases, I only want at most one row with identical
a and b, but I can have identical a and b when c is a real date as long
as that date is also unique for a given a and b.

I'm guessing I'm going to need to use a function and that someone will
yell at me for using NULLs to represent real data, but I thought I'd be
brave and ask anyway, in case I am missing some other solution that
doesn't involve the use of triggers etc.

Cheers,

Bob Edwards.