[OFFTOPIC] Query Help

Someone asked me to create a query for them that would compute some averages
for them over time.

Basically I have a view that looks like this:

sample_request_line_item_number | integer |
sample_origin | character varying |
date_of_request | timestamp with time zone |
username | character varying |
date_shipped | date |
date_difference | double precision |

I have a query that does what I need:

select username, avg(date_difference) from sample_request_average_days where
date_shipped between '2002-11-01' and '2002-11-30' group by username;

Basically it is getting the average number of days it took to ship something
by salesperson, by month.

The problem is that I want it to print the results for each month for which
there are entries (without hard-coding dates).

Is there any easy way to do this that I'm missing?

Any help appreciated. Please reply off-list to minimize noise.

Thanks,
Hunter