DISTINCT ON troubles

Hi,

I have a query that reads:

SELECT DISTINCT ON (messageboard.threadid) messageboard.threadid,
messageboard.topic, owner.ownerid, owner.username FROM messageboard, owner
WHERE messageboard.ownerid=owner.ownerid AND messageboard.leagueid =
'$leagueid' ORDER BY messageboard.messageid DESC LIMIT $entries_on_page
OFFSET $beginThread"

The purpose of this query is to retrieve unique threadid's in the order of
the most recent posts that have been made to each thread.  When I use this
query I get an error that:

"SELECT DISTINCT ON expressions must match initial ORDER BY expressions".

Of course, if I put ORDER BY threadid in the query it would order it in the
order that the thread was created, not in the last post made.

I have tried using GROUP BY threadID, I get a similar order.

Am I just approaching this all wrong and need to create a temporary table
and draw from that, or is there a way to salvage this query?

Thanks so much,
Jeremy