limit results to one row per foreign object

["Alan Bullock" <liststuff(at)gmail(dot)com>]

hi all, I have the following schema:

CREATE TABLE auctions ( id serial NOT NULL, user_id int4, title 
varchar(255), body varchar(255), CONSTRAINT auctions_pkey PRIMARY KEY (id) ) 
WITHOUT OIDS;

CREATE TABLE bids ( id serial NOT NULL, auction_id int4, user_id int4, 
amount float8, created_at timestamp, CONSTRAINT bids_pkey PRIMARY KEY (id) ) 
WITHOUT OIDS;

CREATE TABLE users ( id serial NOT NULL, username varchar(255), CONSTRAINT 
users_pkey PRIMARY KEY (id) ) WITHOUT OIDS;

I'd like to return all the bids for a given auction, but limit it to only 
the *latest* bid from each user. so regardless of how many bids a user has 
placed, only their latest is returned.

I dont have a clue where to even start with this and would appreciate some 
pointers
thanks

alan