How to return rows with 0 count for missing data?

I have unevenly distributed data (wrt date) for a few years (2003-2008). I want to query data for a given set of start and end date, grouping the data by any of the supported intervals (day, week, month, quarter, year) in PostgreSQL 8.3.

The problem is that some of the queries give results continuous over the required period, as this one:

select to_char(date_trunc('month',date), 'YYYY-MM-DD'), count(distinct post_id) 
from some_table
where category_id = 1
and entity_id = 77
and entity2_id = 115 
and date <= '2008-12-06'
and date >= '2007-12-01'
group by date_trunc('month',date)
order by date_trunc('month',date);

  to_char   | count 
------------+-------
 2007-12-01 |    64
 2008-01-01 |    31
 2008-02-01 |    14
 2008-03-01 |    21
 2008-04-01 |    28
 2008-05-01 |    44
 2008-06-01 |   100
 2008-07-01 |    72
 2008-08-01 |    91
 2008-09-01 |    92
 2008-10-01 |    79
 2008-11-01 |    65
(12 rows)

But some of them miss some intervals because there is no data present, as this one:

select to_char(date_trunc('month',date), 'YYYY-MM-DD'), count(distinct post_id) 
from some_table
where category_id=1
and entity_id = 75
and entity2_id = 115 
and date <= '2008-12-06'
and date >= '2007-12-01'
group by date_trunc('month',date)
order by date_trunc('month',date);
    
  to_char   | count 
------------+-------

 2007-12-01 |     2
 2008-01-01 |     2
 2008-03-01 |     1
 2008-04-01 |     2
 2008-06-01 |     1
 2008-08-01 |     3
 2008-10-01 |     2
(7 rows)

where the required resultset is:

  to_char   | count 
------------+-------
 2007-12-01 |     2
 2008-01-01 |     2
 2008-02-01 |     0
 2008-03-01 |     1
 2008-04-01 |     2
 2008-05-01 |     0
 2008-06-01 |     1
 2008-07-01 |     0
 2008-08-01 |     3
 2008-09-01 |     0
 2008-10-01 |