possible race condition in trigger functions on insert operations?

Bit of a trigger NOOB Q:

I am trying to use a trigger function to automatically populate new
rows in a table with a public ID of the form YYYY-NNN such that the
42nd row created in 2011 would get the ID "2011-042". Each row is
associated via an iasid column with a row in an audit table that has a
timestamp column called created. This works OK, but I am worried about
two rows getting the same case_no if they come in at the same time
(whatever that means):

declare
case_yr integer;
yr_case_count bigint;
begin
select date_part('year', created) into case_yr
from audit
where audit.sid = NEW.iasid;

select count(*) into yr_case_count
from fwa_case, audit
where fwa_case.iasid=audit.sid
and date_part('year', created) = case_yr;

NEW.case_no = to_char( case_yr, '9999' ) || '-' ||
to_char(1+yr_case_count, 'FM000');
return NEW;
end;

Do I have to worry about this, or does ACID bail me out? If the
former, what do I do? I am thinking first put a uniqueness constraint
on the column and then figure out how to do retries in a trigger
function.

kenneth