limit results to one row per foreign object

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