How can I get the last element out of GROUP BY sets?

I'm trying to produce summary data from a table (using PGSQL 7.4.1):

CREATE TABLE readings( "when" timestamp, value integer );

The summary will be based on various time periods.  I've been using date_trunc(
'hour', "when" ) and GROUP BY for the min/max/average readings with no problems.
But, one piece of data I need is the last value for each GROUP BY period.  Alas,
I cannot figure out how to do this.

If I wanted to loop from a script, I could, for instance, execute the following
for each GROUP BY period (filling in ? appropriately):

SELECT date_trunc( 'hour', "when" ), value
FROM readings
WHERE date_trunc( 'hour', "when" )::timestamp =  ?
ORDER BY "when" DESC
LIMIT 1

But, I figure there's probably some what to do this in SQL.

Any help?

Thanks,
Rob

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