Another psycopg crash: "ERROR: could not serialize access due to concurrent update"

Here's another problem I'm facing, though this one is less
reproducible.  This sometimes occurs, and sometimes doesn't, which is
odd--especially since I'm not doing anything fancy like threads--just
straightforward loops.  I don't think this is 2.0.10-specific, since
it occasionally happened with earlier versions, but the frequency of
the crash has increased enormously (from about 2% of the time to
50-60% of the time) when I upgraded from 2.0.8 to 2.0.10.

Does the info I included below ring a bell for anyone?

My Setup:

Python 2.4.4 (#1, Jul 31 2008, 18:35:49)
[GCC 3.3.6 (Gentoo 3.3.6, ssp-3.3.6-1.0, pie-8.7.8)] on linux2
libpq on client machine: 8.1.11
postgresql/libpq on db server: 8.2.3

The Error That Doesn't Always Occur:

Traceback (most recent call last):
 File "/sm/merch_queue.py", line 211, in ?
   {'merchantid':merchantid, 'merchemailset_id':merchemailset_id})
psycopg.ProgrammingError: ERROR:  could not serialize access due to
concurrent update
CONTEXT:  SQL statement "SELECT 1 FROM ONLY "public"."prospects" x
WHERE "merchantid" = $1 FOR SHARE OF x"

insert into merch_queue (merchantid, emailnum, merchemailset_id)
                 values ('787595203495998', 2, 72);