Crosstab-style query in pure SQL

TIA all

I have a table containing milestones achieved for projects, marked as
being in particular quarters.

CREATE TABLE milestones (
proj_id int4,
sortorder SERIAL,
qtr int4,
description varchar(200)
);

Now I need the milestone descriptions output across the page like:
proj_id | q1 | q2 | q3 | q4
and sorted according to "sortorder".

Judicious use of CASE can get me the quarterly columns but I need to
eliminate holes. Any clever ideas without resorting to procedural
solutions (either table function or application code)?

TIA

- Richard Huxton